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The dihydride CpRe(PPh3)2H2 (1) catalyzes H/D exchange between C6D6 and other arenes or alkanes. Compound 1 also undergoes photochemical phosphine substitution with PMe3 to give CpRe(PPh3)(PMe3)H2 and then CpRe(PMe3)2H2. Mechanistic studies of these reactions are inconsistent with [CpRe(PPh3)H2] as an intermediate. An alternative mechanism is presented proposing that the active species for H/D exchange is the 14-electron cyclic allyl intermediate [(η3-C5H7)Re(PPh3)2] (E), in which both hydrides have migrated from the rhenium to the cyclopentadienyl ligand. This intermediate accounts for the fact that (1) deuterium does not exchange into the hydride ligands of complex 1 during the H/D exchange catalysis and (2) phosphine substitution occurs by an associative pathway. The precursor to intermediate E, [(η4-C5H6)Re(PPh3)2H] (D), can undergo reversible orthometalation, allowing H/D exchange between the hydride ligands and the ortho phosphine positions. Evidence is presented to support this new mechanism as well as to rule out other feasible mechanisms.

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.

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