CALORIMETRIC STUDIES OF THE HEATS OF PROTONATION OF THE METAL IN CIS-M(CO)2(BIDENTATE PHOSPHINE)2 COMPLEXES OF CHROMIUM, MOLYBDENUM, AND TUNGSTEN

Titration calorimetry has been used to determine the heats of protonation (DELTA-H(HM)) of cis-M(CO)2(L L)2 complexes (M = Cr, Mo, W; L L = dppm, dppe, dppp, arphos, dmpe) with CF3SO3H in 1,2-dichloroethane solvent at 25.0-degrees-C. Spectroscopic studies show that protonation occurs at the metal center to form [M(H)(CO)2(L L)2]CF3SO3 complexes with trans CO groups. For the M(CO)2[Ph2P(CH2)(n)PPh2]2 complexes, DELTA-H(HM) becomes less exothermic as the chelate size increases from n = 1 (-29.7 kcal mol-1) to n = 3 (-19.0 kcal mol-1) for Mo and from n = 1 (-31.5 kcal mol-1) to n = 2 (-25.1 kcal mol-1) for W. The higher basicities of complexes with small chelates are ascribed to distortions imposed on the M(CO)2(L L)2 complexes by the chelate ligand and to reduced steric hindrance in the protonated product. Substituting the dppe chelates in Mo(CO)2(dppe)2 (DELTA-H(HM) = -27.4 kcal mol-1) with dmpe increases metal basicity (DELTA-H(HM) = -38.7 kcal mol-1), and with the arphos ligand metal basicity is decreased (DELTA-H(HM) = -23.8 kcal mol-1). In descending group 6, the basicities (DELTA-H(HM)) of the M(CO)2(dppm)2 complexes increase in the order Cr (-25.5 kcal mol-1) << Mo (-29.7 kcal mol-1) < W (-31.5 kcal mol-1), but for the M(CO)2(dppe)2 complexes metal basicity decreases in the order Mo (-27.4 kcal mol-1) > W (-25.1 kcal mol-1). The group 8 complex, (eta-5-C5Me5)2Os (DELTA-H(HM) = -26.6 kcal mol-1), is substantially more basic than (eta-5-C5Me5)2Ru (DELTA-H(HM) = -19.0 kcal mol-1). The heats of protonation (DELTA-H(HN) of a series of organonitrogen bases have also been determined.