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 (ΔH_HM) of cir-M(Co)₂(L⌒L)₂ complexes (M = Cr, Mo, W; ITL = dppm, dppe, dppp, arphos, dmpe) with CF₃So₃H in 1,2-dichloroethane solvent at 25.0 °C. Spectroscopic studies show that protonation occurs at the metal center to form [M(H)(C0)2(L⌒L)2]CF₃S0₃ complexes with trans CO groups. For the M(Co)₂[Ph₂P(CH₂)_nPPh₂]₂ complexes, ΔH_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 (ΔH_HM =-27.4 kcal mol^-1) with dmpe increases metal basicity (ΔH_HM = -38.7 kcal mol^-1), and with the arphos ligand metal basicity is decreased (ΔH_HM = -23.8 kcal mol^-1). In descending group 6, the basicities (A/7HM) 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, (π⁵-C₅Me₅)20s (ΔH_HM =-26.6 kcal mol^-1), is substantially more basic than (π⁵-C₅Me₅)Ru (ΔH_HM = -19.0 kcal mol^-1).The heats of protonation (ΔH_HN) of a series of organonitrogen bases have also been determined.