This paper provides both experimental and theoretical evidence of the remarkable flexibility of B-0-X bond angles (X = Si or Ge). Single-crystal X-ray diffraction analyses of PhB(OSiPh,),, PhB(OGePh,), and B(0SiP h,), are reported. In the compound B(OSiPh,),, the B-0-SiPh, units are sterically constrained and the three B-0-Si angles are approximately equal [138.5(2), 139.2(2) and 142.5(2)"]. In both PhB(OSiPh,), and PhB(OGePh,),, different angles are observed for the two chemically equivalent B-0-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 H,B-0-XH, (X= Si or Ge), [(HO)H,B-SiH,]-, (HO),B-0-Si(OH), and [(HO)3B-O-Si(OH),]-to ascertain their minimum-energy geometries. The B-0-X angles in H,B-0-XH, (X=Si or Ge) and [(HO)H,B-O-SiH,]-, 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-'. From the calculations, B-0-X angles appear to be as flexible in tetracoordinated boron species as in tricoordinated ones.