Thermodynamics of addition of H-2, CO, N-2, and C-H bonds to M((PPr3)-Pr-i)(2)Cl (M = Ir, Rh) : an unprecedented metal-carbonyl bond strength

The thermodynamics of interconversion of various complexes containing the unit IrL*Cl-2 (L* = P(Pr-i)(3)) have been investigated by calorimetry and equilibrium measurements. These complexes span a wide range of configurations including four- and five-coordinate d(8) (IrL*2ClL', IrL*Cl-2(CO)(2)) and five- and six-coordinate d(6) (IrL*2ClRH and IrL*2ClRH(CO)). On the basis of kinetic experiments, a lower limit to the Ir-Nz bond dissociation enthalpy (BDE) of IrL*Cl-2(N-2) 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 H-2 (48 kcal/mol) and CO (72 kcal/mol) addition to IrL*Cl-2; 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-N-2 BDE of RhL*Cl-2(N-2). Combined with previous calorimetric measurements on rhodium complexes, this value permits the calculation of lower limits to the absolute exothermicities of addition to RhL*Cl-2 for numerous small molecules including H-2, CO, N-2, C2H4, and aldehydic C-H bonds. The results of electronic structure calculations (approximate DFT; PMe3 used to model (PPr3)-Pr-i) 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*Cl-2(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.

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