Thermodynamics of Addition of H2, CO, N2, and C−H Bonds to M(P i Pr3)2Cl (M = Ir, Rh). An Unprecedented Metal−Carbonyl Bond Strength

The thermodynamics of interconversion of various complexes containing the unit IrL*2Cl (L* = P( i Pr)3) have been investigated by calorimetry and equilibrium measurements. These complexes span a wide range of configurations including four- and five-coordinate d8 (IrL*2ClL‘, IrL*2Cl(CO)2) and five- and six-coordinate d6 (IrL*2ClRH and IrL*2ClRH(CO)). On the basis of kinetic experiments, a lower limit to the Ir−N2 bond dissociation enthalpy (BDE) of IrL*2Cl(N2) has been determined (36 kcal/mol). Using this value as an “anchor”, in conjunction with the relative addition enthalpies obtained calorimetrically, it is possible to derive lower limits for the absolute exothermicities of H2 (48 kcal/mol) and CO (72 kcal/mol) addition to IrL*2Cl; estimates can also be made for the addition of benzene and acetylene C−H bonds. These values are unusually high; indeed, the magnitude of the Ir−CO BDE is unprecedented. In addition, kinetic methods have been used to determine a lower limit of 29 kcal/mol to the Rh−N2 BDE of RhL*2Cl(N2). Combined with previous calorimetric measurements on rhodium complexes, this value permits the calculation of lower limits to the absolute exothermicities of addition to RhL*2Cl for numerous small molecules including H2, CO, N2, C2H4, and aldehydic C−H bonds. The results of electronic structure calculations (approximate DFT; PMe3 used to model P i Pr3) are in excellent agreement with the relative experimental enthalpies, while the absolute values calculated for addition to IrL2Cl are significantly greater than the experimentally determined lower limits. Addition of a methane C−H bond is calculated to be significantly less favorable than addition of benzene or acetylene C−H bonds, in accord with the fact that IrL*2Cl(alkyl)H complexes have not been reported. The significant differences in the enthalpies of addition for these three types of C−H bonds are briefly analyzed.