H2 Addition to Pincer Iridium Complexes Yielding trans-Dihydride Products: Unexpected Correlations of Bond Strength with Bond Length and Vibrational Frequencies.

R4PONOP-Ir-Me (R1) and R4POCOP-Ir-CO (R2), R = tBu or iPr, are known to undergo acid-catalyzed oxidative addition of H2 that yields octahedral products with two hydrides in a trans-configuration. We use density functional theory to study the free energies (Δ Gtrans) and equilibrium isotope effects (EIEtrans) for H2/D2 addition to R1, R2, and related complexes for R = tBu, iPr, and Me. For a given R, reaction of R1 is ∼5 kcal/mol more exergonic than R2. For a given subclass of complexes, Δ Gtrans is more exergonic for the smaller R. The computed values of Δ Gtrans vary between +5.1 and -17.4 kcal/mol. EIEtrans varies between 0.78 and 1.22. Counterintuitively, it is the less-exergonic reactions that afford products with shorter Ir-H bonds, greater symmetric and asymmetric trans-Ir-(H)2 stretching vibrational frequencies, and more inverse EIEtrans. This disparity is amplified in Me4PONOP-Os-CO, where Δ Gtrans is -35.2 kcal/mol, yet the Os-H bonds are long, and the Os-H vibrational frequencies are low as compared with the Ir-H bonds, and EIEtrans is high (1.20). Attempts are made to account for the inverted bond strength-bond length correlation based on the hydricity of the products and the total negative charge on the trans-Ir(H)2 unit as computed using the Quantum Theory of Atoms in Molecules.