Crystal and Molecular Structure of Manganese Pentacarbonyl Hydride

Manganese pentacarbonyl hydride, HMn(CO)₅, crystallizes with eight molecules in space group I2/a of the monoclinic system in a cell which at −75° has the dimensions a = 12.18, b = 6.35, c = 19.20 Å., and β= 93.3°. Three-dimensional X-ray data were collected from crystals grown from the liquid phase, and the structure was solved by standard techniques. Although the H atom was not located, its profound influence on the geometry of the molecule can be inferred from the fact that the Mn(CO)₆ portion of the molecule shows insignificant deviations from C_4v symmetry: the five carbon atoms of the carbonyls occupy five of the six corners of a nearly regular octahedron; the manganese atom lies slightly above the basal plane along the fourfold axis toward the apical carbonyl group. This result is in direct contrast to claims from infrared studies that the molecule in the gas phase definitely has symmetry lower than C_4v, and to arguments based on the marked similarities in chemical and physical properties of HMn(CO)₅ and Fe(CO)₅ that HMn(CO)₅ must be a trigonal bipyramid with the H buried in the metal orbitals.