The interplay between metal–metal bonds, four‐electron donor carbonyl groups, and five‐electron donor nitrosyl groups in highly unsaturated binuclear rhenium carbonyl nitrosyls

Optimization of structures for the highly unsaturated binuclear rhenium carbonyl nitrosyls Re2(NO)2(CO)n (n = 5, 4) using the MPW1PW91 and BP86 methods of density functional theory indicate complicated potential energy surfaces with eight singlet and three triplet energetically accessible structures for Re2(NO)2(CO)5 and 11 singlet and six triplet energetically accessible structures for Re2(NO)2(CO)4. The lowest energy structure for Re2(NO)2(CO)5 is a singlet predicted to have two bridging nitrosyl groups and a formal ReRe triple bond to give both rhenium atoms the favored 18-electron configuration. However, other relatively low-energy structures for Re2(NO)2(CO)5 are found with five-electron donor bridging nitrosyl groups, four-electron donor bridging carbonyl groups, and coordinatively unsaturated rheniumatoms. The lowest energy structures for Re2(NO)2(CO)4 are predicted to be unbridged structures having rhenium–rhenium distances short enough for formal triple or even quadruple bonds. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009