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[quadruple bond]Mo. We have arranged these data as a histogram, in order to determine the "normal" range of Mo[quadruple bond]Mo lengths, 2.06-2.17 A, as well as to examine molecules in which the Mo[quadruple bond]Mo length deviates from the norm. We discuss 24 molecules with exceptionally long Mo[quadruple bond]Mo lengths. Another molecule, Mo(2)(pyNC(O)CH(3))(4), was previously reported to have an unusually short Mo[quadruple bond]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)CH(2)CH(3))(4), which displays a completely normal Mo[quadruple bond]Mo bond length of over 2.08 A.
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