Stereochemical and electronic control of M-SO2 bonding geometry in d6 molybdenum and tungsten sulfur dioxide complexes: novel .eta.1 .tautm. .eta.2 sulfur dioxide linkage isomerization in Mo(CO)2(PPh3)2(CNR)(SO2) and structures of Mo(CO)3(P-iso-Pr3)2(SO2) and [Mo(CO)2(py)(PPh3)(.mu.-SO2)]2