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 Mo. We have arranged these data as a histogram, in order to determine the “normal” range of Mo Mo lengths, 2.06−2.17 Å, as well as to examine molecules in which the Mo Mo length deviates from the norm. We discuss 24 molecules with exceptionally long Mo Mo lengths. Another molecule, Mo2(pyNC(O)CH3)4, was previously reported to have an unusually short Mo Mo bond. However, the structure is highly disordered (probably twinned); thus we have prepared and determined the structure of an analogous molecule, Mo2(pyNC(O)CH2CH3)4, which displays a completely normal Mo Mo bond length of over 2.08 Å.