Reduction of sulfur dioxide by (C5R5)M(CO)3H (M = Mo, W; R = H, Me). Chemistry and structures of (C5H5)Mo(CO)3(SO2H), the first example of insertion of SO2 into a metal-hydride bond, and [(C5Me5)Mo(CO)3]2(.mu.-S2O4), a sulfur-bonded dithionite complex