COMPOUNDS CONTAINING B-O-X BONDS (X = SI, GE, SN, PB) .4. CRYSTAL-STRUCTURES OF B(OSIPH3)3, PHB(OSIPH3)2 AND PHB(OGEPH3)2

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)-degrees]. In both PhB(OSiPh3), 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)-degrees and for X = Ge, 130.4(5) and 149.0(5)-degrees]. GAUSSIAN-86 calculations (STO-3G and STO-3G*) were performed on the model compounds H2B-O-XH, (X = Si or Ge), [(HO)H2B-SiH3]-, (HO)2B-O-Si(OH), and [(HO)3B-O-Si(OH)3]- to ascertain their minimum-energy geometries. The B-O-X angles in H2B-O-XH, (X = Si or Ge) and [(HO)H2B-O-SiH3] , could be varied by 22-degrees (36-degrees), 24-degrees (22-degrees) or 16-degrees (28-degrees), 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.