Pincer-ligated (^R4PCP)Ir-based complexes are valuable catalysts for the dehydrogenation of alkanes and alkyl groups. The ligands of all such reported catalysts are substituted with four identical phosphinoalkyl groups (typically t-butyl). The presence of sterically bulky, robust, phosphinoalkyl groups presumably offers protection against cluster formation and bimolecular catalyst deactivation, but is also presumed to raise the barrier of the desired reaction of the fragment with the alkane substrate. However, DFT calculations strongly support the conclusion that the key transition states for alkane dehydrogenation are highly unsymmetrical with respect to their steric demands. For example, the enthalpy calculated for the transition state for b-H elimination by (PCP)Ir(1-butyl)(H), to give (PCP)Ir(1-butene)(H)2, is 12.2 kcal/mol above free (PCP)Ir plus n-butane for the parent species (^tBu4PCP)Ir. By contrast, for the single-methyl-substituted species, (^tBu3MePCP)Ir, the corresponding transition state is only 2.7 kcal/mol above free reactants. The resulting calculated advantage of the (^tBu3MePCP)Ir species (9.5 kcal/mol) is offset only partially (by 6.8 kcal/mol) by tighter binding to 1-butene in the resting state. In this context we have synthesized mixed methyl/t-butyl substituted PCP complexes: [2-(di-tert-butylphosphino)methyl-6-(tert-butyl-methyl-phosphino)methyl]phenyl]tetrahydroiridium, (^tBu3MePCP)IrH4, and [2,6-bis[(tert-butyl-methyl-phosphino)methyl]phenyl]dihydroiridium, (^tBu2Me2PCP)IrH4. We find that these complexes are significantly more effective for alkane dehydrogenation than the parent (^tBu4PCP)Ir complexes under typical conditions. The calculations applied in these systems require an ability to accurately capture steric effects. Accordingly we have tested several DFT methods and protocols against a wide range of experimental data for binding to (^tBu4PCP)Ir. We find that the use of PBE functionals gives significantly better agreement with experiment than is obtained with the commonly used B3LYP functionals.