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 A for the four structures with staggered equatorial CO/CS groups to 3.12 ± 0.04 A 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 A, 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 A 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 A.