Nature of the metal-hydrogen bond in transition metal-hydrogen complexes: Neutron and x-ray diffraction studies of β-pentacarbonylmanganese hydride

An X-ray and neutron diSraction study of a new crystalline form of HMn(CO)s, hereafter designated β-HMn(CO)₅, is described. The material crystallizes with eight molecules in space group C-_2h⁶-C2/c of the monoclinic system in a cell (measured at -75°) of dimensions a = 12.16 (3) Å, b = 6.28 (2) Å, c = 19.34 (4) Å, and β; = 91,2 (5)°. The structural details involving the nonhydrogen atoms derived from 596 visually estimated independent X-ray intensities collected at -75° and from 398 independent neutron diffraction counter data collected at -105° agree within the assigned limits of error. In addition, a precise position for the hydrogen atom has been derived from the neutron data. The β-HMn(CO)₅ molecule deviates only slightly from C4v symmetry. The Mn-C_ax bond length is 1.882 (12) Å, while the average Mn-C_eq, bond length is 1.853 (12) Å (ax = axial, trans to H; eq = equatorial, cis to H). The equatorial carbonyl groups are bent toward the hydrogen atom; the C_ax-Mn-C^eq bond angles range from 94.9 (5) to 98.2 (5)°. The Mn-C O linkages do not deviate significantly from linearity. Thus the molecular structures of α- and β-HMn(CO)₅ are essentially identical, but the crystal structures differ markedly and these differences are dicussed. The Mn-H distance in β-HMn(CO)₃ is 1.601 (16) Å, This value, the first to be determined with such precision in a parent transition metal carbonyl hydride, is consistent with the usual covalent radii sums and with earlier, less accurate X-ray determinations of metal-hydrogen distances, but it is totally inconsistent with some theoretical studies and the various interpretations of the broad-line nmr spectra obtained from this and related compounds. From the present study it is concluded that the hydrogen atom is a stereochemically active ligand and that the metal-hydrogen distance is that of a normal covalent bond.