Low-Energy Low-Spin Unsaturated Binuclear Hexamethylbenzene Rhenium Carbonyls Are Very Different from the Low-Energy High-Spin Unsaturated Binuclear Hexamethylbenzene Manganese Carbonyls

The lowest energy structure for the tetracarbonyl (Me6C6)2Re2(CO)4 has two bridging CO groups and two terminal CO groups with a trans arrangement of the hexamethylbenzene rings. For the unsaturated derivatives (Me6C6)2Re2(CO)n (n = 3, 2, 1), the lowest energy structures are singlet structures with rhenium–rhenium multiple bonds of order 5–n and fully bonded hexahapto η6-Me6C6 rings. This differs from the analogous manganese systems (Me6C6)2Mn2(CO)n for which higher spin-state structures with partially bonded hexamethylbenzene rings are energetically preferred in many cases. The following three types of higher energy (Me6C6)2Re2(CO)3 structures are found: (1) a singlet structure with a four-electron donor η2-μ-CO group and a formal Re–Re single bond; (2) a triplet structure with three bridging μ-CO groups and a formal Re═Re σ + (2/2)π double bond similar to that in dioxygen or the known iron complex (η5-C5H5)2Fe2(μ-CO)3; and (3) a singlet structure of the type (η4-Me6C6)(η6 Me6C6)Re2(μ-CO)3 with one partially bonded tetrahapto hexamethylbenzene ring, three bridging μ-CO groups, and a formal Re≡Re triple bond. The lowest energy structure for the monocarbonyl (Me6C6)2Re2(CO) has a bridging CO group and a very short Re≣Re distance suggesting a formal quadruple bond. However, two triplet (Me6C6)2Re2(CO) structures are found within 5 kcal/mol of this quadruply bonded singlet structure having somewhat longer Re≡Re distances indicative of formal triple bonds.