Density functional theory studies of the series of isomeric d(6) (pentacarbonyl) metal complexes (CO)(5)M(eta(1)-SO2), (nq), (CO)(5)M(eta(1)-OSO)(nq), and (CO)(5)M(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 --> SO2 T-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)(5)M(SO2)(nq) complexes track competition between relative hardness/softness of the metal fragment and its capacity for pi-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.