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(O Et)2, P(OEt)3), (H)4Os(PR3)3 (PR3 = PPhMe2, PPh2Me), HIr(CO)(PPh3)3, a nd CpIr(ER3)(H)2 (ER3 = PPh3, AsPh3, P(OPh)3) complexes with CF3SO3H i n 1,2-dichloroethane solvent at 25.0-degrees-C. For the (H)2Os(PR3)4 c omplexes, 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) incre ases by approximately 15 kcal/mol when two hydride ligands are replace d 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) resul ts in a 10.4 kcal/mol increase in the basicity (-DELTAH(HM)) of the me tal. The basicities of the CpIr(ER3)(H)2 complexes increase in the ord er P(OPh)3 << AsPh3 almost-equal-to PPh3. Comparisons of the basicitie s of CpIr(PPh3)(CO) and (PPh3)2(H)Ir(PPh3)(CO) complexes show that rep lacement 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 gre atly.