Neutron and x-ray diffraction studies of tris(methyldiphenylphosphine)[tetrahydroborato(1-)]copper, Cu[P(C6H5)2CH3]3(BH4). The first accurate characterization of an unsupported metal-hydrogen-boron bridge bond

A single-crystal neutron diffraction study of tris(methyldiphenylphosphine)[tetrahydroborato(1-)]copper, Cu-(PPh2Me)3(BH4), has been carried out at low temperature (15 +- 0.5 K) in order to examine the nature of the unidentate attachment of the tetrahydroborate group to the Cu atom. The compound crystallizes in the space group Pna21, Z = 4. Cell constants at 15 K are a = 19.980 (2) A, b = 10.175 (1) A, and c = 17.282 (2) A. The structure was refined on the basis of measured intensities of 4765 reflections, and final discrepancy factors for all reflections are R(F2) = 0.043 and R(wF2) = 0.047. One H atom of the tetrahydroborate group participates in bonding to the Cu atom with Cu-H = 1.697 (5) A, Cu-B = 2.518 (3) A, and Cu-H-B = 121.7 (4)0. There is no evidence for other Cu-H bonding interactions (the second shortest CuH distance is 2.722 (7) A). The H-Cu-P angles [86.2 (2)0, 108.5 (2)0, 115.3 (2)0] are more widely distributed than the B-Cu-P angles [101.4 (6)0, 101.7 (7)0, 108.8 (7)0], and the bridging H atom is significantly displaced off the pseudothreefold axis defined by the CuP3 group. This bent, off-center positioning of the bridging H atom is reminiscent of similar observations found in M-H-M bridges and may provide some information about the nature of M-H-B overlap. The tetrahydroborate group is somewhat distorted from tetrahedral symmetry; three of the four B-H distances agree well with one another (1.179 +- 0.005 A), but the fourth [1.330 (6) A] is significantly longer. In addition to the neutron diffraction analysis, structures were determined from room-temperature x-ray data obtained from two other crystals. In general, for nonhydrogen structural parameters there is good agreement among all of the determinations including the original work of Atwood et al