LIGAND EFFECTS ON HEATS OF PROTONATION OF MULTIHYDRIDO TRANSITION-METAL COMPLEXES OF OSMIUM AND IRIDIUM

Titration calorimetry has been used to determine heats of protonation (DELTAH(HM)) of the metal in (H)2Os(PR3)4 (PR3 = PPhMe2, PPh2Me, PPh(OEt)2, P(OEt)3), (H)4Os(PR3)3 (PR3 = PPhMe2, PPh2Me), HIr(CO)(PPh3)3, and CpIr(ER3)(H)2 (ER3 = PPh3, AsPh3, P(OPh)3) complexes with CF3SO3H in 1,2-dichloroethane solvent at 25.0-degrees-C. For the (H)2Os(PR3)4 complexes, the basicity (-DELTAH(HM)) increases from 34.2 to 43.3 kcal/mol in the order P(OEt)3 < PPh(OEt)2 < PPh2Me < PPhMe2. The basicity (-DELTAH(HM)) of the metal in (H)4OS(PR3)3 (PR3 = PPhMe2, PPh2Me) increases by approximately 15 kcal/mol when two hydride ligands are replaced by a PR3 ligand to give (H)2Os(PR3)4. Replacement of the two hydride ligands in CpIr(PPh3)(H)2 by a CO ligand to give CpIr(PPh3)(CO) results in a 10.4 kcal/mol increase in the basicity (-DELTAH(HM)) of the metal. The basicities of the CpIr(ER3)(H)2 complexes increase in the order P(OPh)3 << AsPh3 almost-equal-to PPh3. Comparisons of the basicities of CpIr(PPh3)(CO) and (PPh3)2(H)Ir(PPh3)(CO) complexes show that replacement of Cp by the isoelectronic (PR3)2(H) ligand set increases the basicity of the metal. In other complexes, the replacement of Cp* by (PR3)2(H) also increases the metal basicity, but the effect varies greatly.