Structural Characterizations of Salts of HCr(CO)5- and (μ-H)2BH2Cr(CO)4- and Studies of Their Interconversions

Article abstract of DOI:10.1021/ja00389a013

At 0 deg C, BH3 reacted with HCr(CO)5^- in THF to abstract hydride, presumably producing coordinatively unsaturated Cr(CO)5⁰, which immediately aggregated with remaining HCr(CO)5^- to yield the very stable (μ-H)^-.At room temperature two bridging hydride products were obtained.In addition to the binuclear bridging hydride, a second product, (μ-H)2BH2Cr(CO)4^-, resulted from CO loss either prior to or following Cr-H---BH3 adduct formation.The borohydride complex could be reconverted to HCr(CO)5^- on addition of CO; however, (μ-H)^- was also formed in the process.Salts of both title anions were characterized by solution spectroscopic probes as well as X-ray structural analysis.Crystals of were found to belong to the tetragonal space group P4/n, with a = 13.234(2) Angstroem, b = 13.234(2) Angstroem, c = 7.472(2) Angstroem, and Z = 2.R(F) = 3.9percent for 1796 reflections with I > 3?(I).Deep red crystals of <(μ-H)2BH2Cr(CO)4> belong to the triclinic space group P1/, with a = 11.708(3) Angstroem, b = 14.572(6) Andstroem, c = 11.454(3) Angstroem. α = 101.98(3) deg, β = 91.69(2) deg, γ = 77.34(3) deg, and Z = 2.R(F) = 6.6percent for 2880 reflections with I > 3?(I).Most notably, HCr(CO)5^- showed bending of the crystallographically identical equatorial CO groups toward the hydride ligand (<*>(CO)_ax-Cr-(CO)_eq = 95.4(1) deg), as has been exhibited by all monomolecular hydridocarbonyl complexes whose structures are known.Analysis of the ν(CO) infrared spectrum indicated that this pseudooctahedral structure persisted in solution.The hydride ligand induced only a very small trans effect on the Cr-C bond length with Cr-C_trans = 1.852(4) Angstroem and Cr-C_cis = 1.865(3) Angstroem.The hydride ligand was located 1.66(5) Angstroem from Cr.In contrast the Cr-C bonds of (μ-H)2BH2Cr(CO)4^- showed considerable asymmetry with Cr-C_eq(trans to H) = 1.81(1) and 1.82(1) Angstroem and Cr-C_ax(cis to H) = 1.87(2) and 1.85(1) Angstroem.In addition, the axial CO groups bend away from the <(μ-H)2BCr> planar unit, <*>(CO)_ax-Cr-(CO)_ax = 175.6(4) deg, whereas the equatorial CO groups expand into the space made available by the small requirement of the (μ-H)2B bidentate ligand, <*>(CO)_eq-Cr-(CO)_eq = 94.8(4) deg.Carbon-13 NMR spectroscopy showed the CO groups of the (μ-H)2BH2Cr(CO)4^- to be stereochemically rigid at +30 deg C whereas 1H NMR spectroscopy showed rapid interchange of bridging and terminal hydrogens, even at -80 deg C,