The lengths of molybdenum to molybdenum quadruple bonds: Correlations, explanations, and corrections

A systematic search of the Cambridge Crystallographic Database has yielded structures of 467 molecules containing, or reported to contain, a molybdenum to molybdenum quadruple bond, Mo-4-Mo. We have arranged these data as a histogram, in order to determine the "normal" range of Mo-4-Mo lengths, 2.06-2,17 Angstrom, as well as to examine molecules in which the Mo-4-Mo length deviates from the norm. We discuss 24 molecules with exceptionally long Mo-4-Mo lengths. Another molecule, Mo-2(pyNC(O)CH3)(4), was previously reported to have an unusually short Mo-4-Mo bond. However, the structure is highly disordered (probably twinned); thus we have prepared and determined the structure of an analogous molecule, Mo-2(pyNC(O)CH2CH3)(4), which displays a completely normal Mo-4-Mo bond length of over 2.08 Angstrom.