The enthalpy of insertion of sulfur into the metal-hydrogen bond.: Synthetic, structural, and calorimetric study of the complexes HS-M(CO)3C5R5 [M = Cr, Mo, W; R = H, Me]

Synthetic and calorimetric studies of the sulfhydryl complexes HS-M(CO)(3)C5R5 (M = Cr, R = Me; M = Mo, W, R = H, Me) are reported. The Mo and W complexes can be obtained in high yield by reaction of the hydride complexes H-M(CO)(3)C5R5 with Ph3Sb=S, which readily undergoes single S atom transfer to the metal-hydrogen bond yielding the metal-sulfhydryl complex. Direct reaction between the metal hydrides and a limited amount of sulfur also yields the sulfhydryl complexes as the dominant organometallic product. At sulfur atom/metal hydride ratios higher than 1/1, additional products formulated as HSn-M(CO)(3)C5R5 are detected. The enthalpies of insertion of sulfur from Ph3Sb=S and Ss into the metal-hydrogen bond have been determined by solution calorimetry. The HS-M(CO)(3)C5R5 complexes (M = Mo, W) are readily desulfurized by PCy3 for R = H, but not for R = Me. The M-SH bond strength estimates for the complexes HS-M(CO)(3)C5Me5 increases in the order Cr (46) < Mo (55) < W (63) (kcal/mol). The HS-Mo(CO)(3)C5Me5 group has a pK(a) value at least 4 pK(a) units less acidic than that of H-Mo(CO)(3)C5Me5. The crystal structure of HS-W(CO)(3)C5Me5 is reported.