STRUCTURAL CHARACTERIZATIONS OF SALTS OF HCR(CO)5- AND (MU-H)2BH2CR(CO)4- AND STUDIES OF THEIR INTERCONVERSIONS
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At 0 C, BH3 reacted with HCr(CO)5 in THF to abstract hydride, presumably producing coordinatively unsaturated Cr(CO)5, which immediately aggregated with remaining HCr(CO)5 to yield the very stable (-H)[Cr2(CO)10]. At room temperature two bridging hydride products were obtained. In addition to the binuclear bridging hydride, a second product, (-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, (-H)[Cr2(CO)10] 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 [Ph4P][HCr(CO)5] were found to belong to the tetragonal space group P4/n, with a = 13.234 (2) , b = 13.234 (2) , c = 7.472 (2) , and Z = 2. R(F) = 3.9% for 1796 reflections with I > 3(I). Deep red crystals of [PPN] [(-H)2BH2Cr(CO)4] belong to the triclinic space group PI, with a = 11.708 (3) , b = 14.572 (6) , c = 11.454 (3) , = 101.98 (3), = 91.69 (2), = 77.34 (3), and Z = 2. R(F) = 6.6% 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)axCrCCO)eq = 95.4 (1)), as has been exhibited by all mononuclear hydridocarbonyl complexes whose structures are known. Analysis of the v(CO) infrared spectrum indicated that this pseudooctahedral structure persisted in solution. The hydride ligand induced only a very small trans effect on the CrC bond length with CrCtrans = 1.852 (4) and CrCcis = 1.865 (3) . The hydride ligand was located 1.66 (5) from Cr. In contrast the CrC bonds of (-H)2BH2Cr(CO)4 showed considerable asymmetry with CrCeq(trans to H) = 1.81 (1) and 1.82 (1) and CrCax(cis to H) = 1.87 (1) and 1.85 (1) . In addition, the axial CO groups bend away from the [(-H)2BCr] planar unit, (CO)axCr(CO)ax = 175.6 (4), whereas the equatorial CO groups expand into the space made available by the small requirement of the (-H)2B bidentate ligand, (CO)eqCr(CO)eq = 94.8 (4). Carbon-13 NMR spectroscopy showed the CO groups of (-H)2BH2Cr(CO)4 to be stereochemically rigid at +30 C whereas 1H NMR spectroscopy showed rapid interchange of bridging and terminal hydrogens, even at 80 C. 1982, American Chemical Society. All rights reserved.