BH Bond Activation of Trimethylphosphineborane by Transition Metal Complexes: Synthesis of Metal Complexes Bearing Nonsubstituted Boryl−Trimethylphosphine, Cp*M(CO)3(BH2·PMe3) (M = Mo, W)

Photolysis of methylmolybdenum and tungsten complexes, Cp*M(CO) 3 Me (M = Mo, W), in the presence of BH 3 ·PMe 3 ( 1 ) afforded nonsubstituted boryl complexes Cp*M(CO) 3 (BH 2 ·PMe 3 ) ( 2a: M = Mo, 3a: M = W) with concomitant evolution of methane. The structures of 2a and 3a were determined by single-crystal X-ray crystallography. The boryl group in these complexes adopts a tetra-coordinate geometry with coordination of trimethylphosphine to the boron atom. The tungsten−boron bond distance in 3a is significantly longer than those of boryltungsten complexes with a tri-coordinate boryl group. The CO stretching bands in the IR spectra of 2a and 3a were observed at lower wavenumbers than those of the corresponding methyl complexes, indicating the pronounced polarization of the M−B bond, M(−)−B(+). Reactivity of the boryl complexes also featured the polar M−B bond. Boryl complexes having a nonsubstituted cyclopentadienyl ligand, CpM(CO) 3 (BH 2 ·PMe 3 ) ( 2b: M = Mo, 3b: M = W) were generated by the similar photoreaction although they were thermally unstable.