Titration calorimetry has been used to determine the heats of protonation (ΔHHM) 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, ΔHHM 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 (ΔHHM =-27.4 kcal mol-1) with dmpe increases metal basicity (ΔHHM = -38.7 kcal mol-1), and with the arphos ligand metal basicity is decreased (ΔHHM = -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 (ΔHHM =-26.6 kcal mol-1), is substantially more basic than (π5-C5Me5)Ru (ΔHHM = -19.0 kcal mol-1).The heats of protonation (ΔHHN) of a series of organonitrogen bases have also been determined.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry