To evaluate the influence of steric, electronic, and synthetic factors, the synthesis of RuHCl(CO)[P/Pr-2(3,5(CF3)(2)C6H3)](2) was carried out, and its Lewis acidity toward Cl- was compared to that of RuHCl(CO)(P/Pr-3)(2). In this synthesis, Na2CO3 was shown to be a more effective base than NEt3, because Na+ can better mask the nucleophilicity of the potential ligand Cl-. An X-ray structure determination of the hydride-free species RuCl2(CO)(P/Pr2Me)(2) shows it to be a dimer, and this solid-state structure persists in solution, but as several different isomers. The synthesis of RuHCl(CO)(P/Pr2Me)(3) shows that three of this smaller phosphine can crowd around Ru, but dynamic NMR spectra show one phosphine to be weakly bound. The rate of reaction of Me3SiC equivalent to CH with this molecule is suppressed by added free (PPr2Me)-Pr-i, indicating phosphine dissociation to be a mechanistic component.