For all geometry optimizations, we used the M06 functional with the SDD basis set and Stuttgart pseudopotentials for rhodium and 6-31G(d) for all other atoms. This choice of model chemistry is based on a thorough benchmarking: We first tested seventeen functionals and three Rh basis sets that had previ-ously been used in computational studies of rhodium complexes. To determine the best combination, we computed the unsigned deviation for a set of structural parameters (e.g. Rh-Cp * distance, average Rh-P bond length, P-Rh-P bond angle) in a chloride-bound version of the dppb complex relative to its experimental crystal structure (see Figure in the Supporting Information, SI). Across all functionals and parameters, the SDD basis had as small or smaller errors than the other basis sets tested. The M06 functional was chosen due to its small errors for the geometry and prior use to compute accurate energies of rhodium complexes. Consistent with these prior studies, we included dispersion interactions with Grimme's D3 correction and use the larger 6-311++G(d,p) basis set for all nonmetal atoms when evaluating single point energies. As the reactions are conducted in acetonitrile, implicit solvation effects are included in all calculations using the solvation model with density (SMD). Therefore, all energies reported below contain Gibbs free energy terms for the solvent. However, we do not include free energy terms for the solute, thus, we use the symbol E rather than G to avoid confusion. Although the experiments were conducted at 298 K, no temperature effects were taken into account in the calculations (except implicitly in the SMD parametrization ). Atomic charges were computed using the Natural Bond Orbitals (NBO) formalism. All calculations were performed using a development version of GAUSSIAN. 59 the analysis using the analog structures without substituents (see Figure ). We then focus more systematically on individual descriptors by analyzing the trends in the minimal model systems in section 3.3. In Figure , we plot ∆∆E R X of the full complexes. For the dppm, dppb, and dppx complexes, ∆∆E R1 X is negative, thus these complexes all bind hydrogen more favorably than the corresponding bpy complex. Reaction 2 (hydrogen → acetonitrile exchange) is positive for all of the full phosphorus complexes, thus less favorable than for the bpy complex. These results are consistent with the experimental observation that the Rh(III)H species of the phosphorus containing compounds are too stable to react further, while the Rh(III)H bpy complex reacts readily. The ∆∆E R3 X values for the release of acetonitrile depend on the relative magnitude of ∆∆E R1 X and ∆∆E R2 X , as shown in Eq. 4. The results for this reaction vary qualitatively with different ligands, as the dppm and dppb complexes have a negative ∆∆E R3 X while ∆∆E R3 dppx is positive. From just comparing the complexes, it is not clear what is causing the differences in reactivity between the phosphorus and bpy complexes, or the differences among the three phosphorus complexes.