Major Differences Between Mononuclear and Binuclear Manganese Carbonyl Cyanides and Isoelectronic Binary Chromium Carbonyls Arising from Basicity of the Cyanide Nitrogen Atom

Abstract

The manganese carbonyl cyanides Mn(CO)n(CN) and Mn2(CO)n(CN)2 have been investigated by density functional theory. The lowest energy structure for Mn(CO)5(CN) is found to be the experimentally known C-bonded cyanide. The experimentally unknown N-bonded Mn(CO)5(NC) lies ~60 kJ mol–1 above its cyanide isomer. The Mn(CO)4(CN) isomers are obtained by removal of a CO group in various ways from Mn(CO)5(CN) or Mn(CO)5(NC). Three structures, cyanide Mn(CO)3(CN), isocyanide Mn(CO)3(NC), and Mn(CO)3(η2-CN), are found for the tricarbonyl. All low-energy binuclear Mn2(CO)n(CN)2 structures have two end-to-end bridging CN groups. These two η2-CN bridges can be oriented in the same or opposite directions. The Mn2(CO)7(CN)2 structures of this type can be derived from these Mn2(CO)8(CN)2 structures by removal of a CO group with relatively little change in the remainder of the structure. These low-energy Mn2(CO)n(CN)2 structures (n = 8, 7) are very different from the previously studied isoelectronic Cr2(CO)n+2 structures in which low-energy end-to-end CO bridged structures are not found.