A further synthetic route is the reaction of the metal carbonyl with hydrogen.[3] The protonation of metal carbonyl anions, e.g. [Co(CO)4]−, leads also to the formation of metal carbonyl hydrides.
The neutral metal carbonyl hydrides are often volatile and can be quite acidic.[5] The hydrogen atom is directly bounded to the metal. The metal-hydrogen bond length is for cobalt 114 pm, the metal-carbon bond length is for axial ligands 176 and 182 for the equatorial ligands.[6]
Metal carbonyl hydrides are used as catalysts in the hydroformylation of olefins. The catalyst is usually formed in situ in a reaction of a metal salt precursor with the syngas. The hydroformylation starts with the generation of a coordinatively unsaturated 16-electron metal carbonyl hydride complex like HCo(CO)3 or HRh(CO)(PPh3)2 by dissociation of a ligand. Such complexes bind olefins in a first step via π-complexation, thus beginning the transformation of the alkene to the aldehyde.
It has been uncertain for a long time whether metal carbonyl hydrides contain a direct metal-hydrogen bond, although this has been suspected by Hieber for H2Fe(CO)4. The precise structure cannot be identified by X-ray diffraction, particularly the length of a possible metal-hydrogen bond remained uncertain.[7] The exact structure of the metal carbonyl hydrides has been determined by using neutron diffraction and nuclear magnetic resonance spectroscopy.[6][8]
Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, ISBN0-12-352651-5
^Kaesz, H. D.; Saillant, R. B. (1972). "Hydride complexes of the transition metals". Chemical Reviews. 72 (3): 231–281. doi:10.1021/cr60277a003.
^Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (1995). Lehrbuch der anorganischen Chemie (in German). Berlin. ISBN978-3-11-012641-9. OCLC237142268.{{cite book}}: CS1 maint: location missing publisher (link)
^Ralph G. Pearson The Transition-Metal-Hydrogen Bond.Chemical Reviews. volume 85, 1985, S. 41–49, doi:10.1021/cr00065a002.
^ abMcNeill, E. A.; Scholer, F. R. (1977). "Molecular Structure of the Gaseous Metal Carbonyl Hydrides of Manganese, Iron, and Cobalt". Journal of the American Chemical Society. 99 (19): 6243–6249. doi:10.1021/ja00461a011.
^Bau, Robert; Drabnis, Mary H. (1997). "Structures of transition metal hydrides determined by neutron diffraction". Inorganica Chimica Acta. 259 (1–2): 27–50. doi:10.1016/S0020-1693(97)89125-6.
