The reaction enthalpy for O atom transfer from Cp*ReO 3 to PPh 3 in toluene solution is measured to be -28.4 ( 0.3 kcal/mol and -28.0 ( 0.6 kcal/mol in THF solution. This allows the strength of the RedO bond in Cp*ReO 3 (in solution) to be estimated at 116.8
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.
Single-crystal X-ray diffraction studies were conducted on the following compounds: Cp*Ru(PMe 3 ) 2 Cl (1), Cp*Ru(PPhMe 2 ) 2 Cl (2), Cp*Ru(PMePh 2 ) 2 Cl (3), Cp*Ru(PPh 3 ) 2 Cl (4), Cp*Ru(PEt 3 ) 2 Cl (5), Cp*Ru(AsEt 3 ) 2 Cl (6), Cp*Ru(P n Bu 3 ) 2 Cl (7), and Cp*Ru(dmpm)Cl (8). Structural information obtained from these X-ray studies can be correlated with enthalpies of ligand substitution previously determined from solution calorimetry. The cone angle of the phosphine ligand (monodentate) and the Ru-P bond distance were found to be proportional to the enthalpy of reaction.
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