Low-energy high-spin binuclear hexamethylbenzene manganese carbonyl structures with partially bonded benzene rings

Abstract

The geometries and energetics of binuclear hexamethylbenzene manganese carbonyls (Me6C6)2Mn2(CO)n (n = 4, 3, 2, 1), including the experimentally known tetracarbonyl, have been examined using density functional theory. Relatively low- energy structures include trans- and cis-(η6-Me6C6)2Mn2(CO)2(µ-CO)2 doubly bridged tetracarbonyl structures and a triplet (η6-Me6C6)2Mn2(µ-CO)3 triply bridged structure analogous to experimentally known isoelectronic binuclear cyclopentadienyliron carbonyls. However, in addition, a series of low-energy quintet spin state structures (η10–2n-Me6C6)Mn(CO)n–Mn(η6-C6Me6) (n = 4, 3, 2) is found. The tetracarbonyls and tricarbonyl of this series have partially bonded hexamethylbenzene rings suggesting a certain fragility of the hexamethylbenzene-manganese linkage not found in the isoelectronic cyclopentadienyliron compounds. For the dicarbonyl and monocarbonyl (Me6C6)2Mn2(CO)n (n = 2, 1) systems, singlet structures having the triple or quadruple manganese-manganese bonds necessary to give each manganese atom the favored 18-electron configuration are very high-energy structures relative to isomeric triplet and quintet spin state structures.