Cumulated double bond systems as ligands : V. Dialkylsulfurdiimine compounds M(CO)4(RNSNR) (M = Cr,Mo, W) and M(CO)5(RNSNR) (M = Cr,W); structural,spectroscopic and fluxional properties.

The preparation and properties are reported of M(CO)₄(RNSNR) (M = Cr, Mo, W; R = i-Pr, t-Bu), in which the ligand is bidentate and in the trans,trans configuration, and of M(CO)₅(RNSNR) (M = CR, W; R = Et, i-Pr) in which the sulfurdiimine is monodentate and in the cis,trans configuration. In both cases the ligand is linked to the metal atom via the N-atom(s). With M(CO)₅(MeNSNMe) a second isomer is found in which the sulfurdiimine is probably bonded via the S-atom to the metal. All the pentacarbonyl compounds are fluxional; this is attributed to a gliding movement of the metal atom along the NSN system.Both W(CO)₄(t-BuNSN-tBu) and W(CO)₅(MeNSNMe) show vibronic coupling of metal to ligand charge transfer transitions with sulfurdiimine vibrations, as shown with Resonance Raman, but only for W(CO)₅(MeNSNMe) also with the symmetric mode of the equatorial carbonyl groups. The metalsulfurdiimine bond appears to be weak for M(CO)₅(RNSNR), but strong for M(CO)₄(RNSNR).