Chromium Carbonyl Nitrosyls: Comparison with Isoelectronic Manganese Carbonyl Derivatives.

Density functional methods indicate that the global minimum for Cr2(NO)2(CO)8 is a staggered D4d structure in accord with experiment and analogous to the isoelectronic Mn2(CO)10. For the unsaturated Cr2(NO)2(CO)n derivatives the lowest energy structures are very different from the lowest energy structures for the isoelectronic Mn2(CO)n+2 derivatives. Thus the global minimum for Cr2(NO)2(CO)7 is an unbridged structure with a Cr(NO)(CO)4 fragment linked to a Cr(NO)(CO)3 fragment through a CrCr double bond. For Cr2(NO)2(CO)6 the global minimum is a structure with two bridging CO groups, whereas the global minimum for Mn2(CO)8 is an unbridged structure. For Cr2(NO)2(CO)5 both NO groups are bridging NO groups with one of them having a short enough Cr-O distance to be considered a formal five-electron donor η²-μ-NO group. Thus the isoelectronic substitution of NO for CO with a necessary adjustment in the central metal atom can lead to significant shifts in the relative energies of various structural types of metal carbonyl nitrosyls, particularly for unsaturated molecules. For the mononuclear Cr(NO)2-(CO)3 the theoretical structure differs from that deduced from matrix isolation experiments. Moreover, the ν(CO) and ν(NO) vibrational frequencies predicted here for Cr(NO)2(CO)3 correspond more closely with the unassigned species labeled "Cr(NO)(CO)x" in the experiments rather than the species claimed to be Cr(NO)2(CO)3. [ABSTRACT FROM AUTHOR]