PONOP-Ir-Me (1) and POCOP-Ir-CO (2), R = Bu orPr, are known to undergo acid-catalyzed oxidative addition of H that yields octahedral products with two hydrides in a trans-configuration. We use density functional theory to study the free energies (Δ G) and equilibrium isotope effects (EIE) for H/D addition to 1,2, and related complexes for R = Bu,Pr, and Me. For a given R, reaction of 1 is ∼5 kcal/mol more exergonic than2. For a given subclass of complexes, Δ G is more exergonic for the smaller R. The computed values of Δ G vary between +5.1 and -17.4 kcal/mol. EIE 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) stretching vibrational frequencies, and more inverse EIE. This disparity is amplified in PONOP-Os-CO, where Δ G 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 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) unit as computed using the Quantum Theory of Atoms in Molecules.

The tripodal ligated octahedral complex d6-[Rh­(PhB­(CH2PPh2)3)­(H)2(NCMe)] (1-RhH) was discovered by Tejel and co-workers to catalyze the Tishchenko reaction in which two aldehydes are dimerized into an ester. Two fundamentally different mechanisms can be envisaged for this system: (i) an inner-sphere mechanism starting with substitution of the acetonitrile ligand of 1-RhH by an aldehyde and (ii) an outer-sphere mechanism starting with direct insertion of an aldehyde into a Rh–H bond of the intact 1-RhH to make an octahedral Rh-alkoxide intermediate. We use DFT methods to investigate the two mechanisms. The inner-sphere mechanism is computed to be energetically favorable. The outer-sphere one, in contrast, is prohibitively high in energy. This is opposite to catalysis of the same reaction using Gusev’s pincer-ligated octahedral catalyst trans-[(PHNN)­Os­(H)2(CO)] (2-OsH) where the outer-sphere mechanism was previously reported to have very low energy. The different behaviors of 1-RhH and 2-OsH can be attributed to a role from the different metals in the two catalysts as well as a role from their different ligands. Specifically, the higher oxidation state of the metal in 1-RhH, Rh­(III) versus Os­(II), greatly diminishes its thermodynamic hydricity leading to separated ions compared to 2-OsH, whereas the amino functionality of the ligand in 2-OsH greatly favors the kinetic hydricity in the reaction with an aldehyde by hydrogen bonding with the carbonyl group being reduced. Comparisons are also made with Milstein’s trans-[PNN-Ru­(H)2(CO)] alcohol dehydrogenative coupling catalyst (3-RuH) which also lacks the amino functionality.