Binuclear pentalene manganese carbonyl complexes: conventional trans and unconventional cis structures

Abstract

The binuclear pentalene manganese carbonyl trans-(η5,η5-C8H6)Mn2(CO)6 has been synthesized by the reaction of the pentalene dianion with a manganese carbonyl halide. This species as well as its decarbonylation products (C8H6)Mn2(CO) n (n = 5, 4, 3) have been investigated by density functional theory. In all of these optimized structures both pentalene rings are bonded to manganese atoms as pentahapto ligands. The lowest energy (C8H6)Mn2(CO)6 structure is the experimentally observed singlet trans structure but a closely related cis structure lies ∼7 kcal/mol higher. The lowest energy (C8H6)Mn2(CO)5 structure is a singlet structure having a four-electron donor bridging CO group and a Mn–Mn single bond of length ∼2.9 Å. The lowest energy (C8H6)Mn2(CO)4 structure is a singlet structure having all terminal CO groups and a short Mn ≡ Mn triple bond of length ∼2.3 Å. For (C8H6)Mn2(CO)3 low energy structures are found with either triplet or quintet spin states. Low energy structures with pentalene rings only partially bonded to manganese atoms were found for the carbonyl-rich species (C8H6)Mn2(CO) n (n = 7, 8). However, the latter species are predicted not to be viable with respect to carbonyl loss to give the stable (C8H6)Mn2(CO)6.