Binuclear manganesecarbonyl thiocarbonyls: metal–metal multiple bonds versus four-electron donorthiocarbonyl groups

Abstract

Density functional theory (DFT) studies on Mn₂(CS)₂(CO)₈ using the B3LYP and BP86 methods show that no less than eight different unbridged structures are of significantly lower energies than the lowest energy doubly bridged structure. The Mn–Mn single bonds in these Mn₂(CS)₂(CO)₈ structures range from 2.99 ± 0.02 Å for the four structures with staggered equatorial CO/CS groups to 3.12 ± 0.04 Å for the four structures with eclipsed equatorial CO/CS groups. The six lowest energy Mn₂(CS)₂(CO)₇ structures all have four-electron donor bridging η²-μ-CE groups (E = S, O) and formal Mn–Mn single bonds of lengths 2.95 ± 0.01 Å, rather than only two-electron donor CO and CS groups and formal MnMn double bonds. The Mn₂(CS)₂(CO)₇ structures with an η²-μ-CS group are of lower energy than those with an η²-μ-CO group. These Mn₂(CS)₂(CO)₇ structures are similar to the lowest energy structure for Mn₂(CO)₉ predicted previously as well as (Ph₂PCH₂PPh₂)₂Mn₂(CO)₄(η²-μ-CO), which has been synthesized and structurally characterized by X-ray diffraction. The lowest energy Mn₂(CS)₂(CO)₆ structures are predicted have a single four-electron donor bridging η²-μ-CS group and a formal MnMn double bond. However, at only slightly higher energies, Mn₂(CS)₂(CO)₆ structures are found with two η²-μ-CS groups and a formal Mn–Mn single bond. A formal MnMn triple bond of length 2.36 ± 0.03 Å is found in an even higher energy unbridged Mn₂(CS)₂(CO)₆ structure, similar to the lowest energy Mn₂(CO)₈ structure found in a previous theoretical study. The lowest energy structures for Mn₂(CS)₂(CO)₅ have two η²-μ-CS groups and a formal MnMn double bond of length 2.57 ± 0.03 Å.