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 (ΔGtrans) 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, ΔGtrans is more exergonic for the smaller R. The computed values of ΔGtrans 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 ΔGtrans 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.