SUPPORTING INFORMATION. Pyramidanes: The Covalent Form of the Ionic Compounds

SUPPORTING INFORMATION Pyramidanes: The Covalent Form of the Ionic Compounds Vladimir Ya. Lee, 1 * Olga A. Gapurenko, 2 Yuki Ito, 1 Takahiko Meguro, 1 Haruka Sugasawa, 1 Akira Sekiguchi, 1 *, Ruslan M. Minyaev, 2 Vladimir I. Minkin, 2 *Rolfe H. Herber, 3 Heinz Gornitzka 4,5 1 Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305 8571, Japan 2 Institute of Physical and Organic Chemistry, Southern Federal University, Rostov on Don, 344090, Russian Federation 3 Racah Institute of Physics, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel 4 CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France 5 Université de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France *To whom correspondence should be addressed. E-mails: leevya@chem.tsukuba.ac.jp (V. Ya. L.); sekiguch@chem.tsukuba.ac.jp (A. S.); minkin@ipoc.sfedu.ru (V. I. M.). Contents of the Supporting Information: 1. Experimenal Section: details of the X-ray crystallographic analysis for pyramidanes 2, 4, 9, 10a and 10b: Figures S1 S3 (ORTEP drawings) and Tables S1 S15 (tables of the crystallographic data including atomic positional and thermal parameters). Figures S4 S25 (NMR spectral charts of the pyramidanes 2 4, 10, 14 and 15) S2 S53 2. Computational details: geometries and relative energies (Table S16), optimized geometries (Figure S26), NBO data (Table S17), resonance structures (Figure S27), NRT data (Table S18), MO diagram of E[Ge 4 H 4 ] pyramidanes 12'-16' (Figure S28), Sn[Si 4 H 4 ] molecular graph (Figure S29) S54 S61

1. Experimental Section: X-ray crystallography. Table S1. Crystallographic data for germapyramidane2. Identification code Empirical formula gec4_0ma C16 H36 Ge Si4 Formula weight 413.40 Temperature Wavelength Crystal system Space group 150(2) K 0.71073 Å Orthorhombic Pnma Unit cell dimensions a = 11.6135(6) Å α= 90 b = 17.5625(9) Å β= 90 c = 11.4704(6) Å γ= 90 Volume 2339.5(2) Å 3 Z 4 Density (calculated) 1.174 Mg/m 3 Absorption coefficient 1.509 mm -1 F(000) 880 Crystal size 0.47 x 0.35 x 0.17 mm 3 Theta range for data collection 2.12 to 26.40. Index ranges -14<=h<=11, -21<=k<=21, -13<=l<=14 Reflections collected 11688 Independent reflections 2487 [R(int) = 0.0527] Completeness to theta = 26.40 ー 100.0 % Absorption correction Empirical Max. and min. transmission 0.7824 and 0.5386 Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters 2487 / 0 / 113 Goodness-of-fit on F 2 1.082 Final R indices [I>2sigma(I)] R1 = 0.0242, wr2 = 0.0707 R indices (all data) R1 = 0.0254, wr2 = 0.0716 Largest diff. peak and hole 0.337 and -0.784 e.å -3 S2

Figure S1.Crystal structure of the germapyramidane2 (ORTEP plot with the thermal ellipsoids drawn at the 30% probability level), hydrogen atomsare not shown. S3

Table S2.Atomic coordinates ( x 10 4 ) and equivalent isotropic displacement parameters ( ナ 2 x 10 3 )for GeC4_0ma. U(eq) is defined as one third of the trace of the orthogonalized U ij tensor. x y z U(eq) C(1) 2911(2) 7500 3613(2) 17(1) C(2) 2579(1) 6901(1) 4460(1) 17(1) C(3) 2234(2) 7500 5312(2) 18(1) C(4) 2585(2) 6654(1) 1304(1) 44(1) C(5) 4833(2) 7500 1761(2) 49(1) C(6) 3938(1) 5499(1) 3649(2) 35(1) C(7) 1332(1) 5379(1) 4001(2) 34(1) C(8) 2983(2) 5550(1) 6069(1) 35(1) C(9) 330(1) 6655(1) 6628(2) 35(1) C(10) 2156(2) 7500 8027(2) 53(1) Ge(1) 4056(1) 7500 5078(1) 20(1) Si(1) 3257(1) 7500 2016(1) 19(1) Si(2) 2684(1) 5841(1) 4530(1) 22(1) Si(3) 1308(1) 7500 6648(1) 21(1) S4

Table S3.Bond lengths [Å] and angles [ ] for GeC4_0ma. C(1)-C(2)#1 1.4838(17) C(1)-C(2) 1.4839(17) C(1)-Si(1) 1.8751(18) C(1)-Ge(1) 2.1432(17) C(2)-C(3) 1.4906(17) C(2)-Si(2) 1.8675(13) C(2)-Ge(1) 2.1328(13) C(3)-C(2)#1 1.4906(17) C(3)-Si(3) 1.8720(19) C(3)-Ge(1) 2.1325(18) C(4)-Si(1) 1.8661(16) C(4)-H(4A) 0.9800 C(4)-H(4B) 0.9800 C(4)-H(4C) 0.9800 C(5)-Si(1) 1.853(3) C(5)-H(5A) 0.9800 C(5)-H(5B) 0.9800 C(5)-H(5C) 0.9800 C(6)-Si(2) 1.8709(16) C(6)-H(6A) 0.9800 C(6)-H(6B) 0.9800 C(6)-H(6C) 0.9800 C(7)-Si(2) 1.8676(16) C(7)-H(7A) 0.9800 C(7)-H(7B) 0.9800 C(7)-H(7C) 0.9800 C(8)-Si(2) 1.8700(15) C(8)-H(8A) 0.9800 C(8)-H(8B) 0.9800 C(8)-H(8C) 0.9800 C(9)-Si(3) 1.8683(16) S5

C(9)-H(9A) 0.9800 C(9)-H(9B) 0.9800 C(9)-H(9C) 0.9800 C(10)-Si(3) 1.863(2) C(10)-H(10A) 0.9800 C(10)-H(10B) 0.9800 C(10)-H(10C) 0.9800 Ge(1)-C(2)#1 2.1328(13) Si(1)-C(4)#1 1.8661(16) Si(3)-C(9)#1 1.8683(16) C(2)#1-C(1)-C(2) 90.33(14) C(2)#1-C(1)-Si(1) 134.10(7) C(2)-C(1)-Si(1) 134.10(7) C(2)#1-C(1)-Ge(1) 69.32(8) C(2)-C(1)-Ge(1) 69.32(8) Si(1)-C(1)-Ge(1) 129.30(9) C(1)-C(2)-C(3) 89.93(10) C(1)-C(2)-Si(2) 135.89(9) C(3)-C(2)-Si(2) 133.90(9) C(1)-C(2)-Ge(1) 70.07(8) C(3)-C(2)-Ge(1) 69.53(8) Si(2)-C(2)-Ge(1) 115.19(6) C(2)#1-C(3)-C(2) 89.81(14) C(2)#1-C(3)-Si(3) 133.71(7) C(2)-C(3)-Si(3) 133.70(7) C(2)#1-C(3)-Ge(1) 69.56(8) C(2)-C(3)-Ge(1) 69.56(8) Si(3)-C(3)-Ge(1) 132.26(10) Si(1)-C(4)-H(4A) 109.5 Si(1)-C(4)-H(4B) 109.5 H(4A)-C(4)-H(4B) 109.5 Si(1)-C(4)-H(4C) 109.5 S6

H(4A)-C(4)-H(4C) 109.5 H(4B)-C(4)-H(4C) 109.5 Si(1)-C(5)-H(5A) 109.5 Si(1)-C(5)-H(5B) 109.5 H(5A)-C(5)-H(5B) 109.5 Si(1)-C(5)-H(5C) 109.5 H(5A)-C(5)-H(5C) 109.5 H(5B)-C(5)-H(5C) 109.5 Si(2)-C(6)-H(6A) 109.5 Si(2)-C(6)-H(6B) 109.5 H(6A)-C(6)-H(6B) 109.5 Si(2)-C(6)-H(6C) 109.5 H(6A)-C(6)-H(6C) 109.5 H(6B)-C(6)-H(6C) 109.5 Si(2)-C(7)-H(7A) 109.5 Si(2)-C(7)-H(7B) 109.5 H(7A)-C(7)-H(7B) 109.5 Si(2)-C(7)-H(7C) 109.5 H(7A)-C(7)-H(7C) 109.5 H(7B)-C(7)-H(7C) 109.5 Si(2)-C(8)-H(8A) 109.5 Si(2)-C(8)-H(8B) 109.5 H(8A)-C(8)-H(8B) 109.5 Si(2)-C(8)-H(8C) 109.5 H(8A)-C(8)-H(8C) 109.5 H(8B)-C(8)-H(8C) 109.5 Si(3)-C(9)-H(9A) 109.5 Si(3)-C(9)-H(9B) 109.5 H(9A)-C(9)-H(9B) 109.5 Si(3)-C(9)-H(9C) 109.5 H(9A)-C(9)-H(9C) 109.5 H(9B)-C(9)-H(9C) 109.5 Si(3)-C(10)-H(10A) 109.5 S7

Si(3)-C(10)-H(10B) 109.5 H(10A)-C(10)-H(10B) 109.5 Si(3)-C(10)-H(10C) 109.5 H(10A)-C(10)-H(10C) 109.5 H(10B)-C(10)-H(10C) 109.5 C(2)#1-Ge(1)-C(2) 59.12(7) C(2)#1-Ge(1)-C(3) 40.91(5) C(2)-Ge(1)-C(3) 40.91(5) C(2)#1-Ge(1)-C(1) 40.61(4) C(2)-Ge(1)-C(1) 40.61(4) C(3)-Ge(1)-C(1) 58.89(7) C(5)-Si(1)-C(4) 110.16(8) C(5)-Si(1)-C(4)#1 110.16(8) C(4)-Si(1)-C(4)#1 105.46(12) C(5)-Si(1)-C(1) 111.44(10) C(4)-Si(1)-C(1) 109.73(6) C(4)#1-Si(1)-C(1) 109.73(6) C(8)-Si(2)-C(7) 110.17(8) C(8)-Si(2)-C(6) 106.10(8) C(7)-Si(2)-C(6) 109.85(8) C(8)-Si(2)-C(2) 108.90(6) C(7)-Si(2)-C(2) 111.36(7) C(6)-Si(2)-C(2) 110.33(7) C(10)-Si(3)-C(9) 109.38(7) C(10)-Si(3)-C(9)#1 109.38(7) C(9)-Si(3)-C(9)#1 105.11(10) C(10)-Si(3)-C(3) 113.06(10) C(9)-Si(3)-C(3) 109.80(6) C(9)#1-Si(3)-C(3) 109.80(6) Symmetry transformations used to generate equivalent atoms: #1 x,-y+3/2,z S8

Table S4. Bondanisotropic displacement parameters ( ナ 2 x 10 3 ) for GeC4_0ma. The anisotropicdisplacement factor exponent takes the form: -2π 2 [ h 2 a* 2 U 11 +... + 2 h k a* b* U 12 ] U 11 U 22 U 33 U 23 U 13 U 12 C(1) 16(1) 20(1) 15(1) 0-1(1) 0 C(2) 16(1) 21(1) 15(1) 1(1) 0(1) 0(1) C(3) 16(1) 22(1) 14(1) 0 0(1) 0 C(4) 73(1) 36(1) 21(1) -3(1) -5(1) -15(1) C(5) 28(1) 92(2) 27(1) 0 12(1) 0 C(6) 36(1) 32(1) 37(1) -3(1) 5(1) 11(1) C(7) 36(1) 30(1) 37(1) -5(1) 2(1) -10(1) C(8) 47(1) 30(1) 28(1) 10(1) -2(1) 4(1) C(9) 31(1) 35(1) 40(1) 4(1) 14(1) -4(1) C(10) 29(1) 114(3) 15(1) 0 1(1) 0 Ge(1) 16(1) 27(1) 19(1) 0-2(1) 0 Si(1) 23(1) 22(1) 13(1) 0 3(1) 0 Si(2) 25(1) 18(1) 21(1) 1(1) 1(1) 1(1) Si(3) 18(1) 32(1) 14(1) 0 2(1) 0 S9

Table S5. Bondhydrogen coordinates ( x 10 4 ) and isotropic displacement parameters ( ナ 2 x 10 3 )for GeC4_0ma. x y z U(eq) H(4A) 1757 6649 1469 65 H(4B) 2937 6188 1609 65 H(4C) 2706 6682 460 65 H(5A) 4989 7375 944 74 H(5B) 5197 7120 2267 74 H(5C) 5146 8005 1939 74 H(6A) 3822 5636 2829 53 H(6B) 4002 4945 3720 53 H(6C) 4645 5738 3937 53 H(7A) 679 5559 4467 52 H(7B) 1401 4825 4077 52 H(7C) 1207 5512 3180 52 H(8A) 3707 5782 6328 53 H(8B) 3046 4995 6112 53 H(8C) 2353 5723 6572 53 H(9A) 790 6188 6658 53 H(9B) -128 6660 5910 53 H(9C) -184 6675 7304 53 H(10A) 1649 7626 8682 79 H(10B) 2773 7879 7974 79 H(10C) 2492 6995 8150 79 S10

Table S6. Crystallographic data for plumbapyramidane 4. Identification code Empirical formula PbC4first C16 H36 Pb Si4 Formula weight 548.00 Temperature Wavelength Crystal system Space group 150(2) K 0.71073 Å Monoclinic Cm Unit cell dimensions a = 11.7157(19) Å α= 90 b = 17.054(3) Å β= 112.4050(10) c = 6.2809(10) Å γ= 90 Volume 1160.2(3) Å 3 Z 2 Density (calculated) 1.569 Mg/m 3 Absorption coefficient 7.472 mm -1 F(000) 540 Crystal size 0.38 x 0.32 x 0.25 mm 3 Theta range for data collection 2.23 to 26.47. Index ranges -14<=h<=14, -21<=k<=21, -7<=l<=7 Reflections collected 10818 Independent reflections 2315 [R(int) = 0.0280] Completeness to theta = 26.40 ー 99.4 % Absorption correction Empirical Max. and min. transmission 0.2557 and 0.1625 Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters 2315 / 2 / 113 Goodness-of-fit on F 2 1.065 Final R indices [I>2sigma(I)] R1 = 0.0189, wr2 = 0.0460 R indices (all data) R1 = 0.0189, wr2 = 0.0460 Largest diff. peak and hole 1.266 and -1.471 e.å -3 S11

Figure S2.Crystal structure of the plumbapyramidane4 (ORTEP plot with the thermal ellipsoids drawn at the 30% probability level), hydrogen atomsare not shown. S12

Table S7.Atomic coordinates ( x 10 4 ) and equivalent isotropic displacement parameters ( ナ 2 x 10 3 )for PbC4first. U(eq) is defined as one third of the trace of the orthogonalized U ij tensor. x y z U(eq) Pb(1) 719(1) 0-3963(1) 15(1) Si(3) -1930(2) 0-2350(3) 14(1) Si(1) 3447(2) 0 1294(3) 14(1) Si(2) 768(1) 1671(1) 427(2) 15(1) C(2) 763(3) 617(2) -403(7) 13(1) C(4) 4153(7) 0 4496(12) 31(2) C(5) 3996(4) 888(3) 196(8) 24(1) C(10) -2482(4) 898(3) -4197(8) 22(1) C(9) -2598(7) 0-83(13) 26(2) C(3) -207(5) 0-1074(9) 13(1) C(1) 1723(5) 0 230(9) 12(1) C(6) 2156(4) 1871(3) 3118(8) 25(1) C(8) -590(4) 1857(3) 1235(9) 26(1) C(7) 742(5) 2372(3) -1880(8) 28(1) S13

Table S8. Bond lengths [Å] and angles [ ] forpbc4first. Pb(1)-C(1) 2.440(5) Pb(1)-C(3) 2.443(6) Pb(1)-C(2) 2.453(4) Pb(1)-C(2)#1 2.453(4) Si(3)-C(3) 1.867(6) Si(3)-C(9) 1.868(7) Si(3)-C(10) 1.880(4) Si(3)-C(10)#1 1.880(4) Si(1)-C(4) 1.861(7) Si(1)-C(1) 1.870(6) Si(1)-C(5) 1.876(5) Si(1)-C(5)#1 1.876(5) Si(2)-C(7) 1.870(5) Si(2)-C(2) 1.871(4) Si(2)-C(8) 1.872(5) Si(2)-C(6) 1.877(5) C(2)-C(1) 1.479(5) C(2)-C(3) 1.487(5) C(4)-H(4A) 0.9800 C(4)-H(4B) 0.9800 C(4)-H(4C) 0.9800 C(5)-H(5A) 0.9800 C(5)-H(5B) 0.9800 C(5)-H(5C) 0.9800 C(10)-H(10A) 0.9800 C(10)-H(10B) 0.9800 C(10)-H(10C) 0.9800 C(9)-H(9A) 0.9800 C(9)-H(9B) 0.9800 C(9)-H(9C) 0.9800 C(3)-C(2)#1 1.487(5) C(1)-C(2)#1 1.479(5) C(6)-H(6A) 0.9800 C(6)-H(6B) 0.9800 S14

C(6)-H(6C) 0.9800 C(8)-H(8A) 0.9800 C(8)-H(8B) 0.9800 C(8)-H(8C) 0.9800 C(7)-H(7A) 0.9800 C(7)-H(7B) 0.9800 C(7)-H(7C) 0.9800 C(1)-Pb(1)-C(3) 50.69(18) C(1)-Pb(1)-C(2) 35.18(12) C(3)-Pb(1)-C(2) 35.36(12) C(1)-Pb(1)-C(2)#1 35.18(12) C(3)-Pb(1)-C(2)#1 35.36(12) C(2)-Pb(1)-C(2)#1 50.77(18) C(3)-Si(3)-C(9) 111.8(3) C(3)-Si(3)-C(10) 109.02(17) C(9)-Si(3)-C(10) 109.0(2) C(3)-Si(3)-C(10)#1 109.02(17) C(9)-Si(3)-C(10)#1 109.0(2) C(10)-Si(3)-C(10)#1 109.0(3) C(4)-Si(1)-C(1) 111.2(3) C(4)-Si(1)-C(5) 108.9(2) C(1)-Si(1)-C(5) 110.09(18) C(4)-Si(1)-C(5)#1 108.9(2) C(1)-Si(1)-C(5)#1 110.09(18) C(5)-Si(1)-C(5)#1 107.6(3) C(7)-Si(2)-C(2) 113.63(19) C(7)-Si(2)-C(8) 109.7(2) C(2)-Si(2)-C(8) 109.0(2) C(7)-Si(2)-C(6) 109.5(2) C(2)-Si(2)-C(6) 109.6(2) C(8)-Si(2)-C(6) 105.0(2) C(1)-C(2)-C(3) 89.6(3) C(1)-C(2)-Si(2) 133.4(3) C(3)-C(2)-Si(2) 132.8(3) C(1)-C(2)-Pb(1) 71.9(3) S15

C(3)-C(2)-Pb(1) 72.0(3) Si(2)-C(2)-Pb(1) 131.5(2) Si(1)-C(4)-H(4A) 109.5 Si(1)-C(4)-H(4B) 109.5 H(4A)-C(4)-H(4B) 109.5 Si(1)-C(4)-H(4C) 109.5 H(4A)-C(4)-H(4C) 109.5 H(4B)-C(4)-H(4C) 109.5 Si(1)-C(5)-H(5A) 109.5 Si(1)-C(5)-H(5B) 109.5 H(5A)-C(5)-H(5B) 109.5 Si(1)-C(5)-H(5C) 109.5 H(5A)-C(5)-H(5C) 109.5 H(5B)-C(5)-H(5C) 109.5 Si(3)-C(10)-H(10A) 109.5 Si(3)-C(10)-H(10B) 109.5 H(10A)-C(10)-H(10B) 109.5 Si(3)-C(10)-H(10C) 109.5 H(10A)-C(10)-H(10C) 109.5 H(10B)-C(10)-H(10C) 109.5 Si(3)-C(9)-H(9A) 109.5 Si(3)-C(9)-H(9B) 109.5 H(9A)-C(9)-H(9B) 109.5 Si(3)-C(9)-H(9C) 109.5 H(9A)-C(9)-H(9C) 109.5 H(9B)-C(9)-H(9C) 109.5 C(2)-C(3)-C(2)#1 90.0(4) C(2)-C(3)-Si(3) 135.0(2) C(2)#1-C(3)-Si(3) 135.0(2) C(2)-C(3)-Pb(1) 72.7(3) C(2)#1-C(3)-Pb(1) 72.7(3) Si(3)-C(3)-Pb(1) 113.3(2) C(2)-C(1)-C(2)#1 90.7(4) C(2)-C(1)-Si(1) 134.7(2) C(2)#1-C(1)-Si(1) 134.7(2) C(2)-C(1)-Pb(1) 72.9(3) S16

C(2)#1-C(1)-Pb(1) 72.9(3) Si(1)-C(1)-Pb(1) 113.3(3) Si(2)-C(6)-H(6A) 109.5 Si(2)-C(6)-H(6B) 109.5 H(6A)-C(6)-H(6B) 109.5 Si(2)-C(6)-H(6C) 109.5 H(6A)-C(6)-H(6C) 109.5 H(6B)-C(6)-H(6C) 109.5 Si(2)-C(8)-H(8A) 109.5 Si(2)-C(8)-H(8B) 109.5 H(8A)-C(8)-H(8B) 109.5 Si(2)-C(8)-H(8C) 109.5 H(8A)-C(8)-H(8C) 109.5 H(8B)-C(8)-H(8C) 109.5 Si(2)-C(7)-H(7A) 109.5 Si(2)-C(7)-H(7B) 109.5 H(7A)-C(7)-H(7B) 109.5 Si(2)-C(7)-H(7C) 109.5 H(7A)-C(7)-H(7C) 109.5 H(7B)-C(7)-H(7C) 109.5 Symmetry transformations used to generate equivalent atoms: #1 x,-y,z S17

Table S9. Bondanisotropic displacement parameters ( ナ 2 x 10 3 ) for PbC4first. The anisotropicdisplacement factor exponent takes the form: -2π 2 [ h 2 a* 2 U 11 +... + 2 h k a* b* U 12 ] U 11 U 22 U 33 U 23 U 13 U 12 Pb(1) 17(1) 13(1) 15(1) 0 7(1) 0 Si(3) 11(1) 18(1) 14(1) 0 4(1) 0 Si(1) 10(1) 15(1) 17(1) 0 5(1) 0 Si(2) 21(1) 7(1) 19(1) -1(1) 10(1) 1(1) C(2) 13(2) 9(2) 15(2) 1(1) 4(1) 1(1) C(4) 17(3) 49(4) 22(3) 0 3(2) 0 C(5) 24(2) 19(2) 32(2) -4(2) 15(2) -7(2) C(10) 19(2) 23(2) 21(2) 3(2) 5(2) 7(2) C(9) 14(3) 44(4) 22(3) 0 8(3) 0 C(3) 12(2) 10(2) 15(2) 0 4(2) 0 C(1) 12(2) 6(2) 18(3) 0 4(2) 0 C(6) 27(2) 21(2) 26(2) -10(2) 11(2) -5(2) C(8) 28(2) 19(2) 34(2) 1(2) 18(2) 9(2) C(7) 45(3) 13(2) 29(2) 3(2) 19(2) 1(2) S18

Table S10. Bondhydrogen coordinates ( x 10 4 ) and isotropic displacement parameters ( ナ 2 x 10 3 )for PbC4first. x y z U(eq) H(4A) 3935 486 5082 46 H(4B) 5053-37 5006 46 H(4C) 3844-450 5088 46 H(5A) 3393 1312-71 35 H(5B) 4087 758-1251 35 H(5C) 4794 1056 1337 35 H(10A) -2007 1355-3386 33 H(10B) -3359 982-4517 33 H(10C) -2367 824-5650 33 H(9A) -2231 427 1008 39 H(9B) -2420-502 737 39 H(9C) -3494 75-805 39 H(6A) 2908 1758 2842 37 H(6B) 2129 1537 4367 37 H(6C) 2156 2423 3551 37 H(8A) -496 2368 1993 38 H(8B) -636 1446 2289 38 H(8C) -1348 1854-155 38 H(7A) 1436 2259-2342 41 H(7B) 813 2910-1294 41 H(7C) -36 2314-3214 41 S19

Table S11. Crystallographic data for stannatetrasilapyramidane 9. Identification code Empirical formula SnSi4third_0m C36 H84 Si8 Sn Formula weight 860.44 Temperature Wavelength Crystal system 150(2) K 0.71073 Å Triclinic Space group P-1 Unit cell dimensions a = 12.7913(8) Å α= 68.3480(10) b = 12.8192(8) Å β= 87.9350(10) c = 16.8594(11) Å γ = 76.5840(10) Volume 2495.6(3) Å 3 Z 2 Density (calculated) 1.145 Mg/m 3 Absorption coefficient 0.726 mm -1 F(000) 924 Crystal size 0.23 x 0.22 x 0.16mm 3 Theta range for data collection 1.301 to 26.430 Index ranges -16<=h<=16, -16<=k<=16, -21<=l<=21 Reflections collected 34840 Independent reflections 10150 [R(int) = 0.0893] Completeness to theta = 25.242 99.6 % Absorption correction Empirical Max. and min. transmission 0.892and0.850 Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters 10150 / 500 / 794 Goodness-of-fit on F 2 1.085 Final R indices [I>2sigma(I)] R1 = 0.0332, wr2 = 0.0867 R indices (all data) R1 = 0.0341, wr2 = 0.0874 Largest diff. peak and hole 2.095 and -0.835 e.å -3 S20

Figure S3.Crystal structure of the stannatetrasilapyramidane 9 (ORTEP plot with the thermal ellipsoids drawn at the 30% probability level), hydrogen atomsare not shown andonly the main position (86%) of the rotationally disordered structure is depicted for clarity). S21

Table S12.Atomic coordinates ( x 10 4 ) and equivalent isotropic displacement parameters ( ナ 2 x 10 3 )for SnSi4third_0m. U(eq) is defined as one third of the trace of the orthogonalized U ij tensor. x y z U(eq) Sn(1) 5578(1) 8073(1) 6180(1) 26(1) C(34) 9082(2) 9789(3) 8572(2) 48(1) Si(1) 6962(1) 6409(2) 7520(3) 23(1) Si(2) 5197(1) 6352(2) 7689(2) 22(1) Si(3) 4822(1) 8159(2) 7728(3) 22(1) Si(4) 6580(1) 8219(2) 7547(3) 23(1) Si(5) 8555(1) 5173(1) 7327(1) 23(1) Si(6) 4204(1) 5001(1) 7693(1) 25(1) Si(7) 3203(1) 9467(1) 7770(1) 24(1) Si(8) 7568(1) 9608(1) 7447(1) 24(1) C(1) 9691(2) 5926(3) 7136(3) 56(1) C(2) 8409(3) 4835(3) 6328(2) 32(1) C(3) 7402(3) 4422(4) 6302(2) 57(1) C(4) 9392(3) 3939(4) 6242(4) 45(1) C(5) 8328(4) 5974(3) 5543(2) 63(1) C(6) 8889(4) 3880(4) 8391(2) 42(1) C(7) 10023(6) 3116(6) 8408(5) 68(2) C(8) 8059(4) 3158(5) 8576(3) 96(2) C(9) 8936(5) 4353(5) 9099(3) 90(2) C(10) 5200(3) 3612(3) 7783(3) 51(1) C(11) 3469(4) 4622(4) 8718(2) 44(1) C(12) 3159(5) 3464(4) 8941(3) 79(2) C(13) 4157(4) 4537(5) 9462(2) 66(1) C(14) 2414(4) 5520(5) 8664(3) 83(2) C(15) 3350(3) 5519(3) 6651(2) 34(1) C(16) 2689(3) 4647(4) 6684(3) 48(1) C(17) 2590(3) 6706(3) 6455(2) 48(1) C(18) 4127(4) 5586(5) 5912(2) 62(1) C(19) 2058(2) 8766(3) 7795(3) 50(1) S22

C(20) 2946(3) 10848(3) 6769(2) 29(1) C(21) 1992(3) 11773(4) 6873(4) 42(1) C(22) 3926(2) 11363(2) 6537(2) 38(1) C(23) 2640(3) 10549(3) 6013(2) 46(1) C(24) 3267(3) 9663(3) 8839(2) 39(1) C(25) 4013(4) 10412(4) 8842(2) 66(1) C(26) 2114(4) 10146(4) 9072(3) 54(1) C(27) 3693(4) 8470(4) 9548(2) 71(2) C(28) 6644(2) 10895(3) 7584(3) 50(1) C(29) 8159(3) 10126(3) 6344(2) 36(1) C(30) 7197(4) 10844(3) 5685(2) 59(1) C(31) 8751(3) 9141(3) 6084(2) 46(1) C(32) 8934(5) 10894(5) 6315(4) 56(2) C(33) 8578(3) 8899(3) 8420(2) 33(1) C(35) 7949(4) 8369(6) 9205(3) 62(2) C(36) 9470(3) 7937(3) 8332(2) 49(1) Si(1') 6785(8) 6241(11) 7545(18) 28(2) Si(2') 4977(7) 6584(9) 7668(12) 23(2) Si(3') 4965(8) 8349(11) 7687(18) 27(2) Si(4') 6773(8) 8010(12) 7550(20) 31(3) Si(5') 8126(4) 4716(4) 7400(3) 27(1) Si(6') 3638(3) 5634(4) 7552(3) 26(1) Si(7') 3608(4) 9910(4) 7770(3) 32(1) Si(8') 8078(5) 9105(6) 7391(3) 39(1) C(1') 7487(15) 3494(16) 7472(18) 60(6) C(2') 8723(15) 5186(13) 6317(9) 40(4) C(3') 9125(18) 6284(15) 6104(13) 52(5) C(4') 9660(20) 4250(20) 6220(20) 51(8) C(5') 7815(18) 5470(20) 5631(12) 60(6) C(6') 9059(16) 4120(20) 8414(11) 40(6) C(7') 9690(20) 5030(20) 8422(19) 79(8) C(8') 9820(30) 3000(30) 8480(30) 57(9) C(9') 8363(18) 3920(30) 9198(11) 59(7) C(10') 2307(14) 6798(18) 7281(17) 65(5) C(11') 3392(15) 4741(15) 8683(10) 28(4) C(12') 2519(10) 4106(12) 8719(9) 19(3) S23

C(13') 4343(17) 3960(20) 9280(15) 58(5) C(14') 2943(16) 5800(15) 8941(12) 39(4) C(15') 3779(17) 5191(18) 6590(11) 54(5) C(16') 3010(20) 4460(20) 6535(19) 52(6) C(17') 3480(30) 6270(20) 5757(14) 76(6) C(18') 4943(18) 4590(30) 6491(18) 76(6) C(19') 4270(20) 11096(18) 7751(17) 66(5) C(20') 2649(14) 10583(17) 6787(10) 47(5) C(21') 1700(20) 11520(20) 6880(20) 56(9) C(22') 3230(20) 11150(20) 6003(13) 70(6) C(23') 2161(19) 9660(20) 6648(15) 61(5) C(24') 3086(19) 9346(19) 8888(11) 56(5) C(25') 4020(30) 8820(30) 9539(19) 85(8) C(26') 2340(30) 10380(30) 9060(30) 76(9) C(27') 2520(20) 8370(20) 8985(18) 77(6) C(28') 9438(14) 8200(20) 7255(18) 70(6) C(29') 7806(17) 10445(16) 6360(11) 49(5) C(30') 6621(18) 11160(20) 6310(20) 86(7) C(31') 7920(30) 10110(30) 5586(13) 86(9) C(32') 8620(30) 11190(30) 6300(30) 58(10) C(33') 8216(17) 9290(20) 8444(11) 47(4) C(35') 7170(20) 10140(20) 8538(17) 75(6) C(36') 8300(30) 8170(30) 9207(19) 60(7) S24

Table S13. Bond lengths [Å] and angles [ ] forsnsi4third_0m. Sn(1)-Si(2') 2.746(16) Sn(1)-Si(3') 2.75(2) Sn(1)-Si(4) 2.763(4) Sn(1)-Si(3) 2.782(3) Sn(1)-Si(1) 2.786(3) Sn(1)-Si(4') 2.79(3) Sn(1)-Si(1') 2.79(2) Sn(1)-Si(2) 2.801(3) C(34)-C(33') 1.46(2) C(34)-C(33) 1.534(4) Si(1)-Si(4) 2.275(2) Si(1)-Si(2) 2.2796(19) Si(1)-Si(5) 2.3666(15) Si(2)-Si(3) 2.279(2) Si(2)-Si(6) 2.3710(14) Si(3)-Si(4) 2.2732(19) Si(3)-Si(7) 2.3608(15) Si(4)-Si(8) 2.3661(14) Si(5)-C(1) 1.880(3) Si(5)-C(2) 1.914(3) Si(5)-C(6) 1.918(4) Si(6)-C(10) 1.891(3) Si(6)-C(11) 1.892(4) Si(6)-C(15) 1.914(3) Si(7)-C(19) 1.880(3) Si(7)-C(20) 1.912(3) Si(7)-C(24) 1.916(4) Si(8)-C(28) 1.879(3) Si(8)-C(29) 1.921(3) Si(8)-C(33) 1.922(3) C(2)-C(3) 1.509(5) S25

C(2)-C(4) 1.537(4) C(2)-C(5) 1.552(4) C(6)-C(8) 1.516(7) C(6)-C(9) 1.535(5) C(6)-C(7) 1.545(6) C(11)-C(13) 1.513(5) C(11)-C(12) 1.534(5) C(11)-C(14) 1.538(6) C(15)-C(17) 1.528(5) C(15)-C(16) 1.534(4) C(15)-C(18) 1.555(4) C(20)-C(22) 1.523(4) C(20)-C(21) 1.546(4) C(20)-C(23) 1.547(4) C(24)-C(25) 1.503(6) C(24)-C(27) 1.541(5) C(24)-C(26) 1.558(5) C(29)-C(31) 1.518(5) C(29)-C(32) 1.539(5) C(29)-C(30) 1.553(5) C(33)-C(36) 1.522(5) C(33)-C(35) 1.541(5) Si(1')-Si(2') 2.269(9) Si(1')-Si(4') 2.270(12) Si(1')-Si(5') 2.365(8) Si(2')-Si(3') 2.272(12) Si(2')-Si(6') 2.371(8) Si(3')-Si(4') 2.270(10) Si(3')-Si(7') 2.370(8) Si(4')-Si(8') 2.367(8) Si(5')-C(2') 1.891(13) Si(5')-C(1') 1.894(14) Si(5')-C(6') 1.922(15) S26

Si(6')-C(11') 1.886(14) Si(6')-C(15') 1.896(14) Si(6')-C(10') 1.925(15) Si(7')-C(19') 1.895(15) Si(7')-C(20') 1.896(14) Si(7')-C(24') 1.910(15) Si(8')-C(33') 1.897(14) Si(8')-C(29') 1.912(15) Si(8')-C(28') 1.913(15) C(2')-C(3') 1.525(16) C(2')-C(4') 1.536(16) C(2')-C(5') 1.551(17) C(6')-C(8') 1.51(2) C(6')-C(9') 1.541(17) C(6')-C(7') 1.564(18) C(11')-C(13') 1.508(17) C(11')-C(12') 1.514(15) C(11')-C(14') 1.555(17) C(15')-C(16') 1.532(17) C(15')-C(17') 1.543(18) C(15')-C(18') 1.545(18) C(20')-C(22') 1.517(17) C(20')-C(23') 1.545(18) C(20')-C(21') 1.547(16) C(24')-C(25') 1.507(19) C(24')-C(27') 1.538(18) C(24')-C(26') 1.560(17) C(29')-C(31') 1.511(18) C(29')-C(32') 1.545(16) C(29')-C(30') 1.565(18) C(33')-C(36') 1.517(19) C(33')-C(35') 1.559(18) S27

Si(2')-Sn(1)-Si(3') 48.8(3) Si(4)-Sn(1)-Si(3) 48.41(6) Si(4)-Sn(1)-Si(1) 48.42(6) Si(3)-Sn(1)-Si(1) 70.42(8) Si(2')-Sn(1)-Si(4') 71.0(4) Si(3')-Sn(1)-Si(4') 48.4(4) Si(2')-Sn(1)-Si(1') 48.4(3) Si(3')-Sn(1)-Si(1') 70.7(5) Si(4')-Sn(1)-Si(1') 48.0(4) Si(4)-Sn(1)-Si(2) 70.96(7) Si(3)-Sn(1)-Si(2) 48.19(6) Si(1)-Sn(1)-Si(2) 48.16(5) Si(4)-Si(1)-Si(2) 90.30(6) Si(4)-Si(1)-Si(5) 133.47(8) Si(2)-Si(1)-Si(5) 136.17(8) Si(4)-Si(1)-Sn(1) 65.25(12) Si(2)-Si(1)-Sn(1) 66.27(10) Si(5)-Si(1)-Sn(1) 123.85(16) Si(3)-Si(2)-Si(1) 89.53(6) Si(3)-Si(2)-Si(6) 136.67(8) Si(1)-Si(2)-Si(6) 133.60(8) Si(3)-Si(2)-Sn(1) 65.47(11) Si(1)-Si(2)-Sn(1) 65.57(11) Si(6)-Si(2)-Sn(1) 122.35(12) Si(4)-Si(3)-Si(2) 90.36(6) Si(4)-Si(3)-Si(7) 136.15(9) Si(2)-Si(3)-Si(7) 133.11(8) Si(4)-Si(3)-Sn(1) 65.35(12) Si(2)-Si(3)-Sn(1) 66.35(10) Si(7)-Si(3)-Sn(1) 120.82(15) Si(3)-Si(4)-Si(1) 89.80(6) Si(3)-Si(4)-Si(8) 134.16(9) Si(1)-Si(4)-Si(8) 136.04(8) S28

Si(3)-Si(4)-Sn(1) 66.25(12) Si(1)-Si(4)-Sn(1) 66.33(12) Si(8)-Si(4)-Sn(1) 124.96(17) C(1)-Si(5)-C(2) 105.65(16) C(1)-Si(5)-C(6) 107.7(2) C(2)-Si(5)-C(6) 116.66(17) C(1)-Si(5)-Si(1) 109.58(11) C(2)-Si(5)-Si(1) 111.13(13) C(6)-Si(5)-Si(1) 106.01(15) C(10)-Si(6)-C(11) 104.79(19) C(10)-Si(6)-C(15) 107.44(17) C(11)-Si(6)-C(15) 116.67(17) C(10)-Si(6)-Si(2) 107.81(11) C(11)-Si(6)-Si(2) 107.94(16) C(15)-Si(6)-Si(2) 111.66(13) C(19)-Si(7)-C(20) 106.78(16) C(19)-Si(7)-C(24) 107.17(17) C(20)-Si(7)-C(24) 116.16(16) C(19)-Si(7)-Si(3) 108.20(11) C(20)-Si(7)-Si(3) 111.83(13) C(24)-Si(7)-Si(3) 106.39(16) C(28)-Si(8)-C(29) 107.87(17) C(28)-Si(8)-C(33) 106.73(16) C(29)-Si(8)-C(33) 116.57(16) C(28)-Si(8)-Si(4) 109.31(11) C(29)-Si(8)-Si(4) 110.74(16) C(33)-Si(8)-Si(4) 105.40(14) C(3)-C(2)-C(4) 109.3(3) C(3)-C(2)-C(5) 107.6(3) C(4)-C(2)-C(5) 108.0(3) C(3)-C(2)-Si(5) 112.6(2) C(4)-C(2)-Si(5) 111.8(3) C(5)-C(2)-Si(5) 107.3(2) S29

C(8)-C(6)-C(9) 110.0(5) C(8)-C(6)-C(7) 110.0(5) C(9)-C(6)-C(7) 105.8(5) C(8)-C(6)-Si(5) 112.0(3) C(9)-C(6)-Si(5) 107.6(3) C(7)-C(6)-Si(5) 111.2(4) C(13)-C(11)-C(12) 108.0(4) C(13)-C(11)-C(14) 107.1(4) C(12)-C(11)-C(14) 106.1(4) C(13)-C(11)-Si(6) 111.2(3) C(12)-C(11)-Si(6) 111.4(3) C(14)-C(11)-Si(6) 112.7(3) C(17)-C(15)-C(16) 109.1(3) C(17)-C(15)-C(18) 108.9(3) C(16)-C(15)-C(18) 107.7(3) C(17)-C(15)-Si(6) 112.6(2) C(16)-C(15)-Si(6) 110.4(3) C(18)-C(15)-Si(6) 108.0(2) C(22)-C(20)-C(21) 108.9(3) C(22)-C(20)-C(23) 107.9(3) C(21)-C(20)-C(23) 107.7(3) C(22)-C(20)-Si(7) 113.1(2) C(21)-C(20)-Si(7) 111.0(3) C(23)-C(20)-Si(7) 108.0(2) C(25)-C(24)-C(27) 107.5(4) C(25)-C(24)-C(26) 111.2(4) C(27)-C(24)-C(26) 106.5(3) C(25)-C(24)-Si(7) 112.5(2) C(27)-C(24)-Si(7) 108.8(3) C(26)-C(24)-Si(7) 110.0(3) C(31)-C(29)-C(32) 108.6(4) C(31)-C(29)-C(30) 107.9(3) C(32)-C(29)-C(30) 109.3(3) S30

C(31)-C(29)-Si(8) 113.0(2) C(32)-C(29)-Si(8) 111.1(3) C(30)-C(29)-Si(8) 106.9(3) C(36)-C(33)-C(34) 109.1(3) C(36)-C(33)-C(35) 108.2(4) C(34)-C(33)-C(35) 108.5(3) C(36)-C(33)-Si(8) 112.2(2) C(34)-C(33)-Si(8) 111.5(2) C(35)-C(33)-Si(8) 107.2(3) Si(2')-Si(1')-Si(4') 90.1(3) Si(2')-Si(1')-Si(5') 135.3(4) Si(4')-Si(1')-Si(5') 134.6(4) Si(2')-Si(1')-Sn(1) 64.8(6) Si(4')-Si(1')-Sn(1) 65.9(8) Si(5')-Si(1')-Sn(1) 124.4(10) Si(1')-Si(2')-Si(3') 89.9(3) Si(1')-Si(2')-Si(6') 134.1(5) Si(3')-Si(2')-Si(6') 135.0(4) Si(1')-Si(2')-Sn(1) 66.8(7) Si(3')-Si(2')-Sn(1) 65.8(7) Si(6')-Si(2')-Sn(1) 117.3(7) Si(4')-Si(3')-Si(2') 90.0(3) Si(4')-Si(3')-Si(7') 135.4(5) Si(2')-Si(3')-Si(7') 134.6(4) Si(4')-Si(3')-Sn(1) 66.6(8) Si(2')-Si(3')-Sn(1) 65.4(6) Si(7')-Si(3')-Sn(1) 124.0(10) Si(1')-Si(4')-Si(3') 90.0(3) Si(1')-Si(4')-Si(8') 135.1(4) Si(3')-Si(4')-Si(8') 134.8(5) Si(1')-Si(4')-Sn(1) 66.0(8) Si(3')-Si(4')-Sn(1) 65.1(8) Si(8')-Si(4')-Sn(1) 123.1(11) S31

C(2')-Si(5')-C(1') 106.9(9) C(2')-Si(5')-C(6') 119.3(9) C(1')-Si(5')-C(6') 105.0(10) C(2')-Si(5')-Si(1') 111.1(8) C(1')-Si(5')-Si(1') 108.8(7) C(6')-Si(5')-Si(1') 105.3(9) C(11')-Si(6')-C(15') 127.8(8) C(11')-Si(6')-C(10') 99.0(10) C(15')-Si(6')-C(10') 103.1(10) C(11')-Si(6')-Si(2') 105.7(7) C(15')-Si(6')-Si(2') 112.8(7) C(10')-Si(6')-Si(2') 105.4(7) C(19')-Si(7')-C(20') 105.0(10) C(19')-Si(7')-C(24') 105.7(10) C(20')-Si(7')-C(24') 120.9(10) C(19')-Si(7')-Si(3') 108.2(8) C(20')-Si(7')-Si(3') 111.2(8) C(24')-Si(7')-Si(3') 105.2(9) C(33')-Si(8')-C(29') 118.8(10) C(33')-Si(8')-C(28') 106.0(10) C(29')-Si(8')-C(28') 104.6(10) C(33')-Si(8')-Si(4') 106.8(10) C(29')-Si(8')-Si(4') 111.8(9) C(28')-Si(8')-Si(4') 108.3(8) C(3')-C(2')-C(4') 107.4(15) C(3')-C(2')-C(5') 106.0(15) C(4')-C(2')-C(5') 109.0(17) C(3')-C(2')-Si(5') 113.4(11) C(4')-C(2')-Si(5') 113.0(14) C(5')-C(2')-Si(5') 107.7(12) C(8')-C(6')-C(9') 110(2) C(8')-C(6')-C(7') 112(2) C(9')-C(6')-C(7') 106.4(18) S32

C(8')-C(6')-Si(5') 110(2) C(9')-C(6')-Si(5') 108.7(12) C(7')-C(6')-Si(5') 110.6(14) C(13')-C(11')-C(12') 111.1(14) C(13')-C(11')-C(14') 108.8(16) C(12')-C(11')-C(14') 109.8(13) C(13')-C(11')-Si(6') 118.9(14) C(12')-C(11')-Si(6') 112.2(11) C(14')-C(11')-Si(6') 94.7(10) C(16')-C(15')-C(17') 102.6(17) C(16')-C(15')-C(18') 109.5(17) C(17')-C(15')-C(18') 105.1(18) C(16')-C(15')-Si(6') 114.9(14) C(17')-C(15')-Si(6') 110.3(15) C(18')-C(15')-Si(6') 113.4(14) C(22')-C(20')-C(23') 109.7(17) C(22')-C(20')-C(21') 107.7(17) C(23')-C(20')-C(21') 107.3(17) C(22')-C(20')-Si(7') 110.3(14) C(23')-C(20')-Si(7') 110.8(13) C(21')-C(20')-Si(7') 111.0(16) C(25')-C(24')-C(27') 106.2(19) C(25')-C(24')-C(26') 109(2) C(27')-C(24')-C(26') 112.6(19) C(25')-C(24')-Si(7') 109.3(17) C(27')-C(24')-Si(7') 110.8(14) C(26')-C(24')-Si(7') 108.9(17) C(31')-C(29')-C(32') 107(2) C(31')-C(29')-C(30') 106.7(19) C(32')-C(29')-C(30') 111.0(19) C(31')-C(29')-Si(8') 110.8(14) C(32')-C(29')-Si(8') 110.7(18) C(30')-C(29')-Si(8') 110.0(14) S33

C(34)-C(33')-C(36') 107(2) C(34)-C(33')-C(35') 104.5(18) C(36')-C(33')-C(35') 105.8(19) C(34)-C(33')-Si(8') 118.7(12) C(36')-C(33')-Si(8') 112.3(19) C(35')-C(33')-Si(8') 107.9(14) Symmetry transformations used to generate equivalent atoms: S34

Table S14. Bondanisotropic displacement parameters ( ナ 2 x 10 3 ) for SnSi4third_0m. The anisotropicdisplacement factor exponent takes the form: -2π 2 [ h 2 a* 2 U 11 +... + 2 h k a* b* U 12 ] U 11 U 22 U 33 U 23 U 13 U 12 Sn(1) 24(1) 29(1) 26(1) -11(1) 4(1) -6(1) C(34) 45(2) 70(2) 53(2) -39(2) 7(1) -31(2) Si(1) 14(1) 21(1) 38(1) -17(1) 5(1) -2(1) Si(2) 16(1) 17(1) 37(1) -14(1) 4(1) -5(1) Si(3) 15(1) 17(1) 35(1) -14(1) 7(1) -3(1) Si(4) 16(1) 20(1) 38(1) -18(1) 5(1) -6(1) Si(5) 15(1) 23(1) 34(1) -16(1) 3(1) 0(1) Si(6) 23(1) 22(1) 35(1) -14(1) 6(1) -10(1) Si(7) 18(1) 21(1) 32(1) -12(1) 8(1) -1(1) Si(8) 20(1) 21(1) 36(1) -15(1) 4(1) -8(1) C(1) 23(1) 50(2) 113(3) -51(2) 14(2) -11(1) C(2) 35(2) 28(2) 31(1) -16(1) 6(1) 3(1) C(3) 46(2) 79(3) 67(2) -54(2) -4(2) -8(2) C(4) 45(2) 42(2) 52(2) -30(2) 11(2) 5(2) C(5) 85(3) 46(2) 33(2) -7(2) 18(2) 16(2) C(6) 41(2) 42(2) 32(2) -15(1) -4(1) 14(2) C(7) 53(3) 65(4) 60(3) -23(3) -13(3) 37(3) C(8) 86(3) 67(3) 83(3) 35(3) -2(3) -21(3) C(9) 109(4) 90(4) 48(2) -39(2) -29(2) 48(3) C(10) 35(2) 29(2) 93(3) -28(2) 15(2) -9(1) C(11) 46(3) 45(3) 33(2) 0(2) 3(2) -23(2) C(12) 107(4) 79(3) 72(3) -25(2) 37(3) -71(3) C(13) 90(3) 80(3) 29(2) -9(2) 3(2) -43(3) C(14) 75(3) 99(4) 53(2) -22(2) 32(2) 8(3) C(15) 35(2) 47(2) 34(1) -22(1) 7(1) -24(1) C(16) 47(2) 63(3) 58(2) -38(2) 6(2) -33(2) C(17) 51(2) 41(2) 43(2) -4(1) -18(1) -12(1) C(18) 67(3) 111(4) 43(2) -49(2) 24(2) -52(3) C(19) 25(1) 47(2) 90(3) -38(2) 20(2) -14(1) S35

C(20) 24(2) 28(1) 31(1) -11(1) 0(1) 2(1) C(21) 35(2) 29(2) 49(2) -9(2) 4(2) 9(2) C(22) 40(2) 25(1) 41(2) -7(1) 12(1) -4(1) C(23) 40(2) 53(2) 41(2) -21(1) -13(1) 6(1) C(24) 43(2) 36(2) 31(1) -14(1) 8(1) 6(1) C(25) 73(3) 97(3) 53(2) -52(2) 5(2) -27(2) C(26) 45(2) 67(3) 50(2) -35(2) 19(2) 5(2) C(27) 88(3) 60(3) 28(2) -1(2) 17(2) 22(2) C(28) 29(2) 38(2) 99(3) -44(2) 8(2) -8(1) C(29) 43(2) 33(2) 31(1) -2(1) -1(1) -21(1) C(30) 70(3) 45(2) 49(2) 8(2) -18(2) -24(2) C(31) 55(2) 62(2) 33(1) -22(2) 19(1) -29(2) C(32) 71(3) 56(3) 47(2) -7(2) 8(2) -47(3) C(33) 35(2) 43(2) 30(1) -17(1) 2(1) -21(1) C(35) 72(4) 102(4) 29(2) -22(2) 9(2) -58(3) C(36) 44(2) 42(2) 54(2) -14(2) -18(1) -3(1) Si(1') 20(4) 26(4) 41(4) -19(4) 8(4) -2(3) Si(2') 19(4) 24(4) 28(3) -15(4) -3(3) 0(3) Si(3') 22(4) 19(4) 42(4) -15(4) 10(4) -5(3) Si(4') 20(4) 32(6) 44(4) -14(6) 4(5) -11(4) Si(5') 23(2) 25(2) 34(2) -16(2) 7(2) 1(2) Si(6') 12(2) 12(2) 53(3) -13(2) -9(2) 2(2) Si(7') 29(2) 18(2) 51(3) -20(2) 3(2) 1(2) Si(8') 41(3) 53(4) 36(2) -20(2) 9(2) -34(3) C(1') 32(10) 42(10) 125(19) -52(12) 11(10) -10(8) C(2') 59(13) 14(9) 40(7) -17(7) 4(6) 14(7) C(3') 71(14) 34(9) 50(11) -18(8) 30(10) -14(9) C(4') 68(16) 34(11) 47(13) -27(12) 20(11) 10(11) C(5') 76(14) 72(17) 26(7) -16(10) 1(9) -10(11) C(6') 16(8) 46(12) 43(8) 0(9) 1(6) -7(6) C(7') 52(15) 83(16) 110(20) -34(16) -31(12) -23(12) C(8') 41(16) 52(12) 52(18) 3(11) 4(10) 4(9) C(9') 43(12) 100(20) 18(6) -18(10) -4(7) -4(11) C(10') 43(7) 53(8) 97(10) -30(7) -18(7) -3(6) C(11') 12(10) 34(11) 54(9) -33(7) 8(8) -8(6) C(12') 10(5) 10(5) 28(6) 4(4) -3(4) -4(4) S36

C(13') 51(8) 54(8) 68(8) -14(7) -21(7) -21(6) C(14') 44(7) 44(7) 51(7) -33(6) 12(6) -29(6) C(15') 75(13) 54(13) 54(9) -31(7) -3(11) -36(10) C(16') 62(10) 45(9) 54(10) -22(7) -13(8) -13(8) C(17') 89(10) 77(9) 67(7) -21(7) -8(8) -38(7) C(18') 77(8) 86(10) 82(10) -49(8) 16(8) -22(7) C(19') 70(10) 48(7) 95(10) -41(7) 5(7) -15(7) C(20') 29(12) 47(13) 53(9) -18(9) 5(6) 12(7) C(21') 58(16) 26(11) 49(14) 6(10) 15(11) 19(11) C(22') 56(9) 64(9) 62(7) -8(7) 9(7) 12(7) C(23') 57(9) 68(9) 61(9) -35(7) -11(6) 1(7) C(24') 48(13) 76(17) 46(8) -35(11) 16(7) 3(9) C(25') 88(11) 95(12) 62(8) -31(8) -5(8) -1(8) C(26') 76(13) 81(11) 69(12) -40(9) 25(8) -1(8) C(27') 77(10) 71(9) 75(10) -20(7) 28(7) -18(8) C(28') 39(9) 80(14) 120(20) -60(14) 25(11) -25(8) C(29') 61(12) 61(13) 43(8) -17(7) 6(10) -49(8) C(30') 76(9) 81(10) 92(11) -16(8) -12(8) -25(7) C(31') 150(30) 90(20) 38(8) -22(11) 17(15) -80(20) C(32') 76(16) 36(15) 70(19) -13(12) 2(18) -38(16) C(33') 40(12) 67(15) 42(8) -25(7) -4(9) -18(9) C(35') 65(7) 90(10) 76(9) -45(8) 8(8) -4(7) C(36') 67(12) 65(8) 54(8) -19(7) -7(8) -29(7) S37

Table S15. Bondhydrogen coordinates ( x 104) and isotropic displacement parameters ( ナ 2x 10 3)for SnSi4third_0m. x y z U(eq) H(34A) 8511 10412 8629 73 H(34B) 9548 9414 9097 73 H(34C) 9510 10111 8087 73 H(34D) 9060 10518 8091 73 H(34E) 8992 9935 9105 73 H(34F) 9776 9249 8606 73 H(1A) 10330 5447 6997 83 H(1B) 9854 6052 7653 83 H(1C) 9481 6671 6660 83 H(3A) 6774 4985 6368 86 H(3B) 7453 3674 6768 86 H(3C) 7324 4340 5753 86 H(4A) 9364 3910 5671 68 H(4B) 9387 3177 6673 68 H(4C) 10052 4158 6328 68 H(5A) 8997 6231 5517 94 H(5B) 7722 6568 5595 94 H(5C) 8214 5840 5019 94 H(7A) 10565 3554 8384 102 H(7B) 10075 2880 7914 102 H(7C) 10149 2428 8936 102 H(8A) 8034 2861 8120 145 H(8B) 7351 3637 8604 145 H(8C) 8257 2510 9123 145 H(9A) 8937 3738 9658 135 H(9B) 8306 4991 9028 135 H(9C) 9594 4632 9064 135 H(10A) 5641 3316 8318 76 H(10B) 5665 3761 7298 76 H(10C) 4809 3040 7779 76 H(12A) 3813 2844 9046 119 S38

H(12B) 2722 3495 8464 119 H(12C) 2744 3314 9455 119 H(13A) 4269 5301 9376 98 H(13B) 4854 4000 9498 98 H(13C) 3792 4254 9993 98 H(14A) 2060 5278 9205 125 H(14B) 1938 5582 8198 125 H(14C) 2572 6272 8554 125 H(16A) 2146 4640 7110 72 H(16B) 3168 3875 6842 72 H(16C) 2333 4870 6121 72 H(17A) 2200 6949 5902 71 H(17B) 3007 7266 6433 71 H(17C) 2074 6662 6904 71 H(18A) 4517 4804 5971 94 H(18B) 4642 6034 5936 94 H(18C) 3712 5962 5362 94 H(19A) 2060 8163 8360 75 H(19B) 2142 8420 7359 75 H(19C) 1375 9348 7679 75 H(21A) 2205 12063 7290 63 H(21B) 1377 11429 7074 63 H(21C) 1791 12412 6321 63 H(22A) 3754 12052 6011 57 H(22B) 4527 10793 6448 57 H(22C) 4127 11576 7002 57 H(23A) 1967 10290 6121 69 H(23B) 3214 9931 5953 69 H(23C) 2544 11235 5485 69 H(25A) 4097 10398 9422 99 H(25B) 3710 11206 8452 99 H(25C) 4716 10120 8655 99 H(26A) 1662 9610 9110 80 H(26B) 1803 10899 8629 80 H(26C) 2153 10231 9624 80 H(27A) 4416 8125 9423 106 S39

H(27B) 3210 7968 9574 106 H(27C) 3725 8560 10099 106 H(28A) 6386 10668 8164 75 H(28B) 6030 11190 7167 75 H(28C) 7036 11499 7492 75 H(30A) 7462 11099 5109 89 H(30B) 6833 11519 5815 89 H(30C) 6689 10365 5716 89 H(31A) 8966 9452 5495 70 H(31B) 8276 8626 6123 70 H(31C) 9393 8708 6467 70 H(32A) 9559 10435 6710 84 H(32B) 8563 11534 6484 84 H(32C) 9173 11204 5734 84 H(35A) 7640 7782 9124 93 H(35B) 7371 8977 9274 93 H(35C) 8438 8012 9717 93 H(36A) 9927 7555 8861 73 H(36B) 9906 8264 7855 73 H(36C) 9153 7373 8224 73 H(1'1) 7355 3518 6895 90 H(1'2) 6803 3577 7749 90 H(1'3) 7972 2754 7807 90 H(3'1) 8711 6893 5601 77 H(3'2) 9889 6131 5983 77 H(3'3) 9034 6535 6591 77 H(4'1) 9889 4511 5632 76 H(4'2) 9427 3535 6353 76 H(4'3) 10265 4121 6615 76 H(5'1) 7574 6314 5344 90 H(5'2) 7209 5159 5905 90 H(5'3) 8090 5132 5209 90 H(7'1) 9226 5598 8626 118 H(7'2) 9892 5431 7842 118 H(7'3) 10335 4640 8803 118 H(8'1) 10495 3146 8225 86 S40

H(8'2) 9489 2626 8172 86 H(8'3) 9961 2486 9081 86 H(9'1) 8783 3317 9698 88 H(9'2) 7725 3689 9086 88 H(9'3) 8138 4642 9308 88 H(10D) 1875 6666 7785 97 H(10E) 1907 6757 6812 97 H(10F) 2462 7563 7107 97 H(12D) 2731 3586 8406 29 H(12E) 1844 4666 8458 29 H(12F) 2417 3656 9316 29 H(13D) 4126 3746 9871 87 H(13E) 4920 4373 9208 87 H(13F) 4602 3268 9148 87 H(14D) 2547 5564 9462 58 H(14E) 2458 6404 8478 58 H(14F) 3541 6099 9048 58 H(16D) 2670 4779 5956 78 H(16E) 2450 4486 6946 78 H(16F) 3409 3665 6667 78 H(17D) 3893 6823 5746 114 H(17E) 2706 6620 5733 114 H(17F) 3637 6035 5264 114 H(18D) 5154 3838 6965 114 H(18E) 5432 5069 6496 114 H(18F) 4980 4464 5948 114 H(19D) 4019 11355 8218 100 H(19E) 4084 11747 7204 100 H(19F) 5054 10799 7821 100 H(21D) 1974 11987 7139 83 H(21E) 1178 11152 7252 83 H(21F) 1354 12020 6319 83 H(22D) 2743 11442 5493 105 H(22E) 3861 10591 5946 105 H(22F) 3459 11801 6063 105 H(23D) 1863 9956 6059 92 S41

H(23E) 1587 9498 7044 92 H(23F) 2723 8955 6755 92 H(25D) 3776 8820 10095 127 H(25E) 4571 9271 9360 127 H(25F) 4335 8024 9585 127 H(26D) 1583 10391 8961 113 H(26E) 2497 11103 8680 113 H(26F) 2458 10287 9658 113 H(27D) 3059 7683 8996 115 H(27E) 2000 8621 8502 115 H(27F) 2148 8192 9519 115 H(28D) 9983 8199 7649 105 H(28E) 9647 8530 6666 105 H(28F) 9381 7406 7381 105 H(30D) 6467 11267 6851 129 H(30E) 6124 10742 6201 129 H(30F) 6531 11914 5843 129 H(31D) 7704 10800 5068 129 H(31E) 7460 9577 5629 129 H(31F) 8672 9725 5561 129 H(32D) 8851 11472 5717 87 H(32E) 9244 10723 6695 87 H(32F) 8274 11849 6460 87 H(35D) 7145 10113 9126 113 H(35E) 6539 9900 8406 113 H(35F) 7155 10922 8142 113 H(36D) 8703 8192 9683 91 H(36E) 8670 7523 9051 91 H(36F) 7574 8086 9379 91 S42

Figure S4. 1 H NMR spectrum of the germapyramidane 2 in C 6 D 6. Figure S5. 13 C NMR spectrum of the germapyramidane 2 in C 6 D 6. S43

Figure S6. 29 Si NMR spectrum of the germapyramidane 2 in C 6 D 6. Figure S7. 1 H NMR spectrum of the stannapyramidane 3 in C 6 D 6. S44

Figure S8. 13 C NMR spectrum of the stannapyramidane 3 in C 6 D 6. Figure S9. 29 Si NMR spectrum of the stannapyramidane 3 in C 6 D 6. S45

Figure S10. 119 Sn NMR spectrum of the stannapyramidane 3 in C 6 D 6. Figure S11. 1 H NMR spectrum of the plumbapyramidane 4 in C 6 D 6. S46

Figure S12. 13 C NMR spectrum of the plumbapyramidane 4 in C 6 D 6. Figure S13. 29 Si NMR spectrum of the plumbapyramidane 4 in C 6 D 6. S47

Figure S14. 207 Pb NMR spectrum of the plumbapyramidane 4 in C 6 D 6. Figure S15. 1 H NMR spectrum of the stannatetrasilapyramidane 10 in C 6 D 6. S48

Figure S16. 13 C NMR spectrum of the stannatetrasilapyramidane 10 in C 6 D 6. Figure S17. 29 Si NMR spectrum of the stannatetrasilapyramidane 10 in C 6 D 6. S49

Figure S18. 119 Sn NMR spectrum of the stannatetrasilapyramidane 10 in C 6 D 6. Figure S19. 1 H NMR spectrum of the pentagermapyramidane 14 in C 6 D 6. S50

Figure S20. 13 C NMR spectrum of the pentagermapyramidane 14 in C 6 D 6. Figure S21. 29 Si NMR spectrum of the pentagermapyramidane 14 in C 6 D 6. S51

Figure S22. 1 H NMR spectrum of the stannatetragermapyramidane 15 in C 6 D 6. Figure S23. 13 C NMR spectrum of the stannatetragermapyramidane 15 in C 6 D 6. S52

Figure S24. 29 Si NMR spectrum of the stannatetragermapyramidane 15 in C 6 D 6. Figure S25. 119 Sn NMR spectrum of the stannatetragermapyramidane 15 in C 6 D 6. S53

2. Computations. Table S16. Calculation data (B3LYP/Def2TZVP): number of imaginary frequencies (λ), relative energies with zero-point correction ( E ZPE, kcal mol -1 ), pyramidal (E E') and basal (E' E') bond lengths (Å), buckling distances a (BD, Å), folding angles (FA) of pyramidanes and corresponding distorted structures. Structure Pyramidane (C 4v or C 4 ) Distorted structure (C 2v or C 2 ) λ E ZPE E E' E' E' λ E ZPE BD FA E E' E' E' 5' C[C 4 H 4 ] 0 1.647 1.446 5" C[C 4 (SiH 3 ) 4 ] 0 1.658 1.464 5 C[C 4 (SiMe 3 ) 4 ] 1 2.4 1.660 1.472 0 0.0 0.01 1.20 1.651 / 1.661 1.471 6' Si[C 4 H 4 ] 0 2.046 1.455 6" Si[C 4 (SiH 3 ) 4 ] 0 2.047 1.474 6 Si[C 4 (SiMe 3 ) 4 ] 1 0.7 2.042 1.485 0 0.0 0.01 0.36 2.039 / 2.043 1.484 / 1.486 2' Ge[C 4 H 4 ] 0 2.160 1.456 2" Ge[C 4 (SiH 3 ) 4 ] 0 2.163 1.474 2 Ge[C 4 (SiMe 3 ) 4 ] 0 2.157 1.485 3' Sn[C 4 H 4 ] 0 2.364 1.457 3" Sn[C 4 (SiH 3 ) 4 ] 0 2.365 1.475 3 Sn[C 4 (SiMe 3 ) 4 ] 0 2.359 1.486 4' Pb[C 4 H 4 ] 0 2.464 1.456 4" Pb[C 4 (SiH 3 ) 4 ] 0 2.465 1.474 4 Pb[C 4 (SiMe 3 ) 4 ] 1 0.3 2.460 1.485 0 0.0 0.00 0.003 2.460 1.485 / 1.486 7' C[Si 4 H 4 ] 1 14.5 1.992 2.229 0 0.0 0.75 35.03 1.810 / 2.459 2.331 7" C[Si 4 (SiH 3 ) 4 ] 1 8.1 2.002 2.243 0 0.0 0.67 32.31 1.822 / 2.378 2.319 7"' C[Si 4 (SiMe 3 ) 4 ] 1 6.3 2.019 2.247 0 0.0 0.64 31.23/31.21 1.835 / 2.360 2.315 / 2.317 7 C[Si 4 (SiMe t Bu 2 ) 4 ] 1 5.2 2.050 2.250 0 0.0 0.65 31.37/31.36 1.856 / 2.378 2.320 8' Si[Si 4 H 4 ] 3 4.9 2.525 2.242 0 0.0 0.58 28.23 2.333 / 2.846 2.298 8" Si[Si 4 (SiH 3 ) 4 ] 1 1.3 2.530 2.263 0 0.0 0.39 19.64 5.385 / 2.735 2.289 8"' Si[Si 4 (SiMe 3 ) 4 ] 1 0.3 2.529 2.272 0 0.0 0.25 12.62 2.433 / 2.651 2.282 / 2.283 8 Si[Si 4 (SiMe t Bu 2 ) 4 ] 0 2.542 2.284 9' Ge[Si 4 H 4 ] 3 4.8 2.612 2.246 1 0.0 0.56 27.68 2.417 / 2.922 2.299 9" Ge[Si 4 (SiH 3 ) 4 ] 1 1.1 2.617 2.267 0 0.0 0.38 18.88 2.474 / 2.812 2.290 9"' Ge[Si 4 (SiMe 3 ) 4 ] 1 0.02 2.615 2.267 0 0.0 0.24 11.98 2.521 / 2.729 2.285 / 2.286 9 Ge[Si 4 (SiMe t Bu 2 ) 4 ] 0 2.627 2.288 10' Sn[Si 4 H 4 ] 3 4.5 2.814 2.252 1 0.0 0.54 26.63 2.625 / 3.109 2.303 10" Sn[Si 4 (SiH 3 ) 4 ] 3 0.6 2.815 2.273 0 0.0 0.34 16.69 2.689 / 2.984 2.292 10"' Sn[Si 4 (SiMe 3 ) 4 ] 0 2.812 2.282 10 Sn[Si 4 (SiMe t Bu 2 ) 4 ] 0 2.821 2.301 11' Pb[Si 4 H 4 ] 3 4.4 2.897 2.254 1 0.0 0.53 25.98 2.712 / 3.183 2.303 11" Pb[Si 4 (SiH 3 ) 4 ] 3 0.4 2.898 2.275 1 0.0 0.32 15.92 2.776 / 3.057 2.292 11"' Pb[Si 4 (SiMe 3 ) 4 ] 0 2.895 2.284 11 Pb[Si 4 (SiMe t Bu 2 ) 4 ] 1 0.8 2.900 2.304 0 0.0 0.07 3.34 2.872 / 2.928 2.304 / 2.306 S54

12' C[Ge 4 H 4 ] 1 25.6 2.170 2.348 0 0. 0 0.89 39.40 1.896 / 2.667 2.498 12" C[Ge 4 (SiH 3 ) 4 ] 1 17.4 2.184 2.379 0 0.0 0.82 36.66 1.911 / 2.610 2.502 12"' C[Ge 4 (SiMe 3 ) 4 ] 1 13.3 2.196 2.382 0 0.0 0.80 35.68/35.64 1.924 / 2.587 2.492 / 2.495 12 C[Ge 4 (SiMe t Bu 2 ) 4 ] 1 12.6 2.238 2.381 0 0.0 0.83 36.91/36.82 1.943 / 2.618 2.492 / 2.497 13' Si[Ge 4 H 4 ] 3 16.0 2.626 2.360 0 0.0 0.77 34.62 2.376 / 3.054 2.467 13" Si[Ge 4 (SiH 3 ) 4 ] 3 7.7 2.630 2.390 0 0.0 0.64 29.11 2.410 / 2.970 2.461 13"' Si[Ge 4 (SiMe 3 ) 4 ] 2 4.1 2.628 2.400 0 0.0 0.55 25.48/25.46 2.434 / 2.910 2.450 / 2.452 13 Si[Ge 4 (SiMe t Bu 2 ) 4 ] 1 2.1 2.642 2.409 0 0.0 0.53 24.43 2.456 / 2.902 2.454 14' Ge[Ge 4 H 4 ] 3 15.0 2.703 2.362 0 0.0 0.74 33.65 2.457 / 3.115 2.464 14" Ge[Ge 4 (SiH 3 ) 4 ] 3 7.0 2.707 2.392 0 0.0 0.61 27.97 2.494 / 3.031 2.457 14"' Ge[Ge 4 (SiMe 3 ) 4 ] 1 3.7 2.706 2.403 0 0.0 0.52 24.05/24.03 2.518 / 2.967 2.447 / 2.449 14 Ge[Ge 4 (SiMe t Bu 2 ) 4 ] 1 2.3 2.714 2.413 0 0.0 0.49 22.59/22.58 2.541 / 2.953 2.451 / 2.452 15' Sn[Ge 4 H 4 ] 3 14.9 2.891 2.366 1 0.0 0.72 32.68 2.653 / 3.290 2.465 15" Sn[Ge 4 (SiH 3 ) 4 ] 3 6.4 2.891 2.396 0 0.0 0.59 26.90 2.689 / 3.202 2.458 15"' Sn[Ge 4 (SiMe 3 ) 4 ] 3 2.7 2.886 2.409 0 0.0 0.49 22.49/22.47 2.713 / 3.130 2.447 / 2.450 15 Sn[Ge 4 (SiMe t Bu 2 ) 4 ] 1 1.8 2.895 2.420 0 0.0 0.43 19.90 2.742 / 3.102 2.449 / 2.454 16' Pb[Ge 4 H 4 ] 3 14.6 2.968 2.367 1 0.0 0.70 31.95 2.735 / 3.356 2.464 16" Pb[Ge 4 (SiH 3 ) 4 ] 3 6.0 2.968 2.397 1 0.0 0.57 26.13 2.772 / 3.267 2.456 16"' Pb[Ge 4 (SiMe 3 ) 4 ] 3 2.5 2.965 2.407 2 0.0 0.46 21.56/21.54 2.798 / 3.194 2.445 / 2.447 16 Pb[Ge 4 (SiMe t Bu 2 ) 4 ] 4 1.4 2.971 2.425 0 0.0 0.43 20.01/19.98 2.813 / 3.177 2.459 / 2.463 a distance between the centers of two diagonals of the distorted E' 4 -base. S55

Figure S26. Optimized geometries of the selected pyramidanes: bond lengths are in Å, hydrogen atoms are not shown. S56

Table S17. NBO data (B3LYP/Def2TZVP). Structure Natural charge Wiberg bond index Polarization coefficients on atoms in pyramidal bonds Apical Basal atom Pyramidal Basal Apical Basal atom atom bond bond atom 5' C[C 4 H 4 ] 0.16 0.20 0.70 1.10 0.65 0.76 5 C[C 4 (SiMe 3 ) 4 ] 0.16 0.42 0.68 1.08 0.65 / 0.66 0.76 / 0.75 5 C[C 4 (SiMe 3 ) 4 ] (C 2v ) 0.16 0.43 / 0.42 0.68 1.08 0.65 / 0.66 0.76 / 0.75 6' Si[C 4 H 4 ] +0.59 0.39 0.53 1.17 0.52 / 0.54 0.85 / 0.84 6 Si[C 4 (SiMe 3 ) 4 ] +0.71 0.64 0.48 1.15 a 6 Si[C 4 (SiMe 3 ) 4 ] (C 2v ) +0.71 0.65 / 0.64 0.49 / 0.48 1.15 2' Ge[C 4 H 4 ] +0.62 0.40 0.51 1.19 0.51 / 0.54 0.86 / 0.84 2 Ge[C 4 (SiMe 3 ) 4 ] +0.73 0.65 0.47 1.16 3' Sn[C 4 H 4 ] +0.60 0.39 0.52 1.19 0.51 / 0.55 0.86 / 0.83 3 Sn[C 4 (SiMe 3 ) 4 ] +0.76 0.65 0.45 1.17 0.52 / 0.48 0.86 / 0.88 4' Pb[C 4 H 4 ] +0.57 0.38 0.52 1.20 0.56 0.83 4 Pb[C 4 (SiMe 3 ) 4 ] +0.75 0.64 0.45 1.17 0.48/ 0.52 0.88 / 0.85 4 Pb[C 4 (SiMe 3 ) 4 ] (C 2v ) +0.75 0.64 0.45 1.17 0.52 0.85 7' C[Si 4 H 4 ] 1.41 +0.45 0.78 0.92 0.82 0.57 7' C[Si 4 H 4 ] (C 2v ) 1.09 +0.22 / +0.55 1.03 / 0.66 0.84 0.85 / 0.67 0.53 / 0.74 7 C[Si 4 (SiMe t Bu 2 ) 4 ] 1.46 +0.20 0.73 0.95 0.81 / 0.82 0.58 / 0.58 7 C[Si 4 (SiMe t Bu 2 ) 4 ] (C 2v ) 1.30 +0.06 / +0.31 0.94 / 0.65 0.87 0.85 / 0.80 0.53 / 0.60 8' Si[Si 4 H 4 ] 0.07 +0.09 0.69 1.05 0.64 / 0.65 0.76 / 0.76 8' Si[Si 4 H 4 ] (C 2v ) +0.08 +0.03 / +0.12 0.86 / 0.58 0.99 0.56 / 0.60 0.83 / 0.80 8 Si[Si 4 (SiMe t Bu 2 ) 4 ] 0.12 0.15 0.67 1.05 0.65/ 0.63 0.76 / 0.77 9' Ge[Si 4 H 4 ] 0.04 +0.08 0.68 1.07 0.64 / 0.65 0.77 / 0.76 9' Ge[Si 4 H 4 ] (C 2v ) +0.09 +0.03 / +0.12 0.83 / 0.57 1.01 0.59 / 0.55 0.80 / 0.83 9 Ge[Si 4 (SiMe t Bu 2 ) 4 ] 0.12 0.14 0.67 1.06 0.65 / 0.65 0.75 / 0.76 10' Sn[Si 4 H 4 ] +0.12 +0.05 0.65 1.09 0.61 / 0.62 0.79 / 0.78 10' Sn[Si 4 H 4 ] (C 2v ) +0.27 0.03 / +0.10 0.76 / 0.54 1.04 0.56 / 0.53 0.83 / 0.85 10 Sn[Si 4 (SiMe t Bu 2 ) 4 ] +0.20 0.22 0.61 1.08 0.60 / 0.61 0.80 / 0.79 11' Pb[Si 4 H 4 ] +0.17 +0.04 0.63 1.10 0.60 / 0.62 0.80 / 0.79 11' Pb[Si 4 H 4 ] (C 2v ) +0.31 0.03 / +0.08 0.73 / 0.52 1.06 0.55 / 0.51 0.83 / 0.86 11 Pb[Si 4 (SiMe t Bu 2 ) 4 ] +0.26 0.22 0.59 1.09 0.59 / 0.60 0.81 / 0.80 11 Pb[Si 4 (SiMe t Bu 2 ) 4 ] (C 2v ) +0.27 0.23 / 0.21 0.60 / 0.58 1.09 0.60 / 0.58 0.80 / 0.81 12' C[Ge 4 H 4 ] 1.22 +0.36 0.76 0.90 0.78 / 0.79 0.62 / 0.61 12' C[Ge 4 H 4 ] (C 2v ) 0.96 +0.20 / +0.49 1.10 / 0.59 0.77 0.84 / 0.75 0.55 / 0.66 12 C[Ge 4 (SiMe t Bu 2 ) 4 ] 1.24 +0.12 0.73 0.93 0.78 / 0.79 0.62 / 0.61 12 C[Ge 4 (SiMe t Bu 2 ) 4 ] (C 2v ) 1.10 +0.01 / +0.26 1.01 / 0.59 0.80 0.77 / 0.63 0.63 / 0.77 13' Si[Ge 4 H 4 ] 0.06 +0.06 0.68 1.01 0.64 / 0.64 0.76 / 0.77 13' Si[Ge 4 H 4 ] (C 2v ) +0.17 0.02 / +0.12 0.92 / 0.52 0.92 0.59 / 0.55 0.81 / 0.84 13 Si[Ge 4 (SiMe t Bu 2 ) 4 ] 0.09 0.18 0.66 1.02 0.64 / 0.65 0.77 / 0.76 13 Si[Ge 4 (SiMe t Bu 2 ) 4 ] (C 2v ) +0.02 0.25 / 0.12 0.81 / 0.55 0.97 0.59 / 0.55 0.81 / 0.83 14' Ge[Ge 4 H 4 ] 0.04 +0.05 0.68 1.02 0.63 / 0.64 0.77 / 0.76 14' Ge[Ge 4 H 4 ] (C 2v ) +0.16 0.02 / +0.12 0.88 / 0.51 0.94 0.58 / 0.62 0.81 / 0.78 14 Ge[Ge 4 (SiMe t Bu 2 ) 4 ] 0.10 0.17 0.66 1.02 0.64 / 0.65 0.77 / 0.76 14 Ge[Ge 4 (SiMe t Bu 2 ) 4 ] (C 2v ) 0.01 0.23 / 0.11 0.78 / 0.56 0.98 0.59 / 0.56 0.81 / 0.83 S57

15' Sn[Ge 4 H 4 ] +0.17 +0.003 0.64 1.05 0.61 / 0.60 0.79 / 0.80 15' Sn[Ge 4 H 4 ] (C 2v ) +0.39 0.11 / +0.09 0.79 / 0.48 0.97 0.54 / 0.58 0.84 / 0.82 15 Sn[Ge 4 (SiMe t Bu 2 ) 4 ] +0.19 0.25 0.60 1.05 0.59/ 0.61 0.80 / 0.79 15 Sn[Ge 4 (SiMe t Bu 2 ) 4 ] (C 2v ) +0.28 0.32 / 0.18 0.68 / 0.52 1.02 0.54 / 0.51 0.84 / 0.86 16' Pb[Ge 4 H 4 ] +0.22 0.01 0.62 1.06 0.59 / 0.61 0.81 / 0.79 16' Pb[Ge 4 H 4 ] (C 2v ) +0.43 0.11 / +0.08 0.75 / 0.46 0.99 0.53 / 0.56 0.84 / 0.82 16 Pb[Ge 4 (SiMe t Bu 2 ) 4 ] +0.27 0.26 0.59 1.06 0.58 / 0.60 0.81 / 0.80 16 Pb[Ge 4 (SiMe t Bu 2 ) 4 ] (C 2v ) +0.36 0.31 / 0.20 0.66 / 0.50 1.03 0.52 / 0.49 0.85 / 0.87 a no natural orbital corresponding to this bond S58

........ E E E E H H H H H H H H H H H H H H H H E[C 4 H 4 ] 5 : E = C 21.69 21.69 21.69 21.69 6 : E = Si 19.68 19.68 19.68 19.68 2 : E = Ge 22.87 22.87 22.87 22.87 3 : E = Sn 19.41 19.41 17.63 17.63 E[Si 4 H 4 ] 8 : E = Si 19.68 19.68 19.68 19.68 9 : E = Ge 20.39 20.39 20.39 20.39 10 : E = Sn 20.65 20.65 20.65 20.65 E[Ge 4 H 4 ] 12 : E = C 12.48 12.48 12.48 12.48 13 : E = Si 17.69 17.69 17.69 17.69 14 : E = Ge 18.32 18.32 18.32 18.32 15 : E = Sn 18.72 18.72 18.72 18.72.. Pb H H H H 4 : 36.75........ C C C C C H Si Si Si Si H H H H Si Si Si Si H H H H Si Si Si Si H H H H Si Si H H H H Si Si Si Si H H H 7 : 17.78 17.52 5.25 4.33 4.33 Figure 27. Resonance structures and their NRT weights (%). S59

Table S18. NRT bond orders and bond ionic characters in model pyramidanes (B3LYP/Def2TZVP ). Structure Pyramidal bond Basal bond b EE' % ionic b E'E' % ionic 5' C[C 4 H 4 ] 0.7360 28.8 0.9915 1.7 6' Si[C 4 H 4 ] 0.7310 54.5 1.0099 2.0 2' Ge[C 4 H 4 ] 0.7279 55.9 1.0111 2.0 3' Sn[C 4 H 4 ] 0.6105 0.7340 57.8 51.7 1.1093 1.0259 1.8 3.3 4' Pb[C 4 H 4 ] 0.7643 37.5 1.4509/1.0909 0.7/8.3 7' C[Si 4 H 4 ] 0.8696/0.8196/ 38.5/60.7/52.9 0.9831/0.9519 5.7/2.8 0.5880 8' Si[Si 4 H 4 ] 0.7392 30.8 0.9700 2.4 9' Ge[Si 4 H 4 ] 0.7312 30.9 0.9772 2.8 10' Sn[Si 4 H 4 ] 0.7314 36.9 0.9789 2.5 11' Pb[Si 4 H 4 ] a 12' C[Ge 4 H 4 ] 0.7030 32.4 0.9415 4.9 13' Si[Ge 4 H 4 ] 0.7375 32.0 0.9383 2.0 14' Ge[Ge 4 H 4 ] 0.7313 31.8 0.9441 2.3 15' Sn[Ge 4 H 4 ] 0.7316 39.0 0.9501 2.2 16' Pb[Ge 4 H 4 ] a Basic NRT calculations did not give any resonance structure for Pb[E 4 H 4 ] (E = Si, Ge) pyramidanes. S60

Figure S28. Comparison of the cage bonding MO of the model H-substituted Ge-based pyramidanes E[Ge 4 H 4 ] (12 : E = C; 13 : E = Si; 14 : E = Ge; 15 : E = Sn; 16 : E = Pb). Figure S29. Molecular graph of the model stannasilapyramidane Sn[Si 4 H 4 ]: bond critical points are shown in purple, ring critical points in yellow, cage critical point in blue. S61