Five-electron donor bridging thionitrosyl groups in unsaturated binuclear manganese carbonyl derivatives

Metal carbonyl thionitrosyls are potentially accessible by reactions of metal carbonyl anions and related nucleophiles with N3S3Cl3 as illustrated by the reported synthesis of (η5-C5H5)Cr(NS)(CO)2 from N3S3Cl3 and NaCr(CO)3(η5-C5H5). Related manganese carbonyl thionitrosyls provide the opportunity to examine the behavior of the NS ligand in both mononuclear and binuclear systems that have a reasonable chance of experimental realization by related synthetic methods. In this connection the two low energy structures for Mn2(NS)2(CO)7 are a doubly NS-bridged structure and an unbridged (OC)5Mn–Mn(NS)2(CO)2 structure. A triply bridged Mn2(NS)2(CO)7 structure analogous to the experimental triply bridged Fe2(CO)9 structure is not found at competitive energies. The lowest energy structures for the unsaturated Mn2(NS)2(CO)n (n = 6, 5) all have a five-electron donor bridging η2-NS group, showing the propensity of a bridging NS group to bond to metals through both the nitrogen and sulfur atoms. The lowest energy Mn2(NS)2(CO)6 structure of this type is a particularly attractive synthetic objective since it lies at least 15 kcal/mol below any other Mn2(NS)2(CO)6 structures.