Comparison of M-S, M-O, and M-((eta)62-SO) Structures and Bond Dissociation Energies in d6 (CO)5M(SO2)nq Complexes Using Density Functional Theory

Density functional theory studies of the series of isomeric d6 (pentacarbonyl)metal complexes (CO)5M((eta)^1-SO2)nq, (CO)5M( 1OSO)nq, and (CO)5M((eta)^2-SO2)nq (M = Ti-Hf, nq = 2-; M = V-Ta, nq = 1-; M = Cr -W, nq = 0; M = Mn-Re, nq = 1+; M = Fe-Os, nq = 2+) provide accurate structural modeling and quantitative prediction of the relative stabilities of the isomers. The (eta)^1-S-bound complexes display planar SO2 moieties that adopt staggered orientations with respect to the carbonyl ligands, in keeping with experimental observations. The OSO chain in the (eta)^1-O-bound complexes generally adopts the u-shape with a staggered orientation. The dianions (CO)5(Ti-Hf)( (eta)^1-OSO)2- differ in that the OSO chain adopts the eclipsed z-shape orientation. The (eta) ^2-SO2 complexes exhibit a facial interaction and are stable only for anionic and neutral complexes, supporting the view that this motif involves substantial M (right arrow)SO2 (pi)-back-bonding. The relative stabilities of the isomers generally follow u-shaped trends both across a row and down a family. This fits with qualitative ideas that the bond dissociation energies (BDEs) for the (CO)5M(SO2)nq complexes track competition between relative hardness/softness of the metal fragment and its capacity for -back-bonding. Quantitatively, examination of BDEs by bond energy decomposition approaches suggests that electrostatic considerations dominate bonding for the (eta)^1-SO2 complexes and covalent effects dominate for the (eta)^2-SO2 species, while both are important for (eta)^1 -OSO complexes.