Compounds containing B–O–X bonds (X = Si, Ge, Sn, Pb). Part 4.—Crystal structures of B(OSiPh3)3, PhB(OSiPh3)2and PhB(OGePh3)2

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Abstract
This paper provides both experimental and theoretical evidence of the remarkable flexibility of B–O–X bond angles (X = Si or Ge). Single-crystal X-ray diffraction analyses of PhB(OSiPh3)2, PhB(OGePh3)2 and B(OSiPh3)3 are reported. In the compound B(OSiPh3)3, the B–O–SiPh3 units are sterically constrained and the three B–O–Si angles are approximately equal [138.5(2), 139.2(2) and 142.5(2)°]. In both PhB(OSiPh3)2 and PhB(OGePh3)2, different angles are observed for the two chemically equivalent B–O–X fragments [for X = Si, 139.8(4) and 157.9(3)° and for X = Ge, 130.4(5) and 149.0(5)°]. GAUSSIAN-86 calculations (STO-3G and STO-3G*) were performed on the model compounds H2B–O–XH3(X = Si or Ge), [(HO)H2B–SiH3]–, (HO)2B–O–Si(OH)3 and [(HO)3B–O–Si(OH)3]– to ascertain their minimum-energy geometries. The B–O–X angles in H2B–O–XH3(X = Si or Ge) and [(HO)H2B–O–SiH3]–, could be varied by 22°(36°), 24°(22°) or 16°(28°), respectively [data from STO-3G (and STO-3G*) calculations] without changing the compounds' minimum energies by >5 kJ mol–1. From the calculations, B–O–X angles appear to be as flexible in tetracoordinated boron species as in tricoordinated ones.

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