Calorimetric studies of the heats of protonation of the metal in cis-M(CO)2(bidentate phosphine)2 complexes of chromium, molybdenum, and tungsten

Abstract

Titration calorimetry has been used to determine the heats of protonation ([Delta]H[sub HM]) of cis-M(CO)[sub 2](L-L)[sub 2] complexes (M = Cr, Mo, W; L-L = dppm, dppe, dppp, arphos, dmpe) with CF[sub 3]SO[sub 3]H in 1,2 dichloroethane solvent at 25C. Spectroscopic studies show that protonation occurs at the metal center to form [M(H)(CO)[sub 2](L-L)[sub 2]]CF[sub 3]SO[sub 3] complexes with trans CO groups. For the M(CO)[sub 2][Ph[sub 2]P(CH[sub 2])[sub n]PPh[sub 2]][sub 2] complexes, [Delta]H[sub HM] becomes less exothermic as the chelate size increases from n = 1 ([minus]29.7 kcal mol[sup [minus]1]) to n = 3 ([minus]19.0 kcal mol[sup [minus]1]) for Mo and from n = 1 ([minus]31.5 kcal mol[sup [minus]1]) to n = 2 ([minus]25.1 kcal mol[sup [minus]1]) for W. The higher basicities of complexes with small chelates are ascribed to distortions imposed on the M(CO)[sub 2](L-L)[sub 2] complexes by the chelate ligand and to reduced steric hindrance in the protonated product. Substituting the dppe chelates in Mo(CO)[sub 2](dppe)[sub 2] with dmpe increases metal basicity, and with the arphos ligand metal basicity is decreased ([Delta]H[sub HM] = [minus]23.8 kcal mol[sup [minus]1]). In descending group 6, the basicities ([Delta]H[sub HM]) of the M(CO)[sub 2](dppm)[sub 2] complexes increase in the order Cr W. The group 8 complex, ([eta][sup 5]-C[sub 5]Me[sub 5])[sub 2]Os is substantially more basic than ([eta][sup 5]-C[sub 5]Me[sub 5])[sub 2]Ru. The heats of protonation ([Delta]H[sub HN]) of a series of organonitrogen bases have also been determined.« less