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 (?G_trans) and equilibrium isotope effects (EIE_trans) 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, ?G_trans is more exergonic for the smaller R. The computed values of ?G_trans vary between +5.1 and -17.4 kcal/mol. EIE_trans 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 EIE_trans. This disparity is amplified in ^Me4PONOP-Os-CO, where ?G_trans 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 EIE_trans 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.