Photolysis of [M(CO)6] (M = Cr, W) in the presence of BH3·L (L = NMe3, PMe3, PPh3) gave isolable borane complexes [M(CO)5(η1-BH3·L)] (1a, M = Cr, L = PMe3; 1b, M = Cr, L = PPh3; 1c, M = Cr, L = NMe3; 2a, M = W, L = PMe3; 2b, M = W, L = PPh3; 2c, M = W, L = NMe3). In products 1 and 2, the monoborane−Lewis base adduct coordinates to the metal center through a B−H−M three-center two-electron bond, which was confirmed by X-ray structural analyses of 1a, 2a, and 2b at low temperature. The X-ray crystal structural analysis of 1c at ambient temperature also showed the same coordination mode, although the positions of hydrogen atoms on the boron were not determined. The 1H NMR spectra of 1 and 2 exhibit only one BH signal at −2 to −3 ppm with an intensity of 3H in the temperature range of −80 °C to room temperature. This indicates that the coordinated BH and terminal BH's are rapidly exchanging in solution even at low temperature. When [Mo(CO)6] was used as a precursor, the formation of the corresponding molybdenum−borane complexes, [Mo(CO)5(η1-BH3·L)] (3a, L = PMe3; 3b, L = PPh3; 3c, L = NMe3), was observed by NMR spectroscopy, but the complexes could not be isolated because of their thermal instability. Complexes of pyridineborane [M(CO)5(η1-BH3·NC5H5)] (1d, M = Cr; 2d, M = W) were also observable by NMR spectroscopy. Fenske−Hall MO calculations for the model compound [Cr(CO)5(η1-BH3·PH3)] (1e) demonstrated that the bonding between the borane and metal can be described as donation of the bonding electron pair of BH to the a1 orbital of [Cr(CO)5], and that π back-donation from the metal d orbital to the antibonding σ* orbital of BH is negligible. Compounds 1−3 can be regarded as model compounds of the methane complex [M(CO)5(CH4)], which is observed in the photolyses of [M(CO)6] in methane matrixes. Structural and spectroscopic features of the ligated borane are discussed and compared with those of related compounds.

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