Propionitrile complexes fac-[M(CO)(3)(P-P)(NCEt)] (M = Mo (3), W (4); P-P = Ph2PCH2PPh2 (a), Ph2PC2H4PPh2 (b), Ph2PC3H6PPh2 (c), (S,S)-Ph2PCHMeCHMePPh2 (d), Fe(C5H4PPh2)(2) (e)) were synthesized from [M(CO)(3)(NCEt)(3)] and the corresponding diphosphine. Reactions of 3 and 4 with sulfur dioxide initially gave complexes fac-[M(CO)(3)(P-P)(eta(1)-SO2)] (M = Mo (5), W (6)), which slowly isomerized to mer-[M(CO)(3)(P-P)(eta(1)-SO2)] (M = Mo (7), W (8)). The structures of 7b and 8b were determined by X-ray crystallography. Both compounds are isostructural (monoclinic, space group P2(1)/n (No. 14)) with almost identical unit cell dimensions (7b, a = 14.511 (s) (A) under circle, b = 12.797(2) (A) over circle, c = 16.476(6) (A) over circle, beta = 115.92(2)degrees; 8b, a = 14.478(8) (A) over circle, b = 12,794(3) (A) over circle, c = 16.442(g) (A) over circle, beta = 116.01(2)degrees) and molecular geometries. Treatment of either fac-[M(CO)(3)(P-P)(eta(2)SO(2))] or mer-[M(CO)(3)(P-P)(W-SO2)] with diazomethane yielded the sulfene complexes mer-[M(CO)(3)(P-P)(eta(2)-CH2SO2)] (M = Mo (9), W (10)). The structure of 10a was determined crystallographically: monoclinic, space group P2(1)/n (No. 14), a = 11.719(2) (A) over circle, b = 17.392(4) (A) over circle, c = 13.441(3) (A) over circle, beta = 95.58(2)degrees. The tungsten atom resides in the center of a distorted pentagonal bipyramid. The sulfene ligand occupies two adjacent equatorial sites with the bond distances W-C, 2.322(13) (A) over circle, W-S, 2.353(3) (A) over circle, and S-C, 1,721(12) (A) over circle. The latter equals the S-C single bond distance in thiirane S,S-dioxide, indicating a high degree of charge density transfer into the LUMO of the sulfene ligand.