Titration calorimetry has been used to determine the heats of protonation ([delta]H[subscript] HM) of transition metal complexes. These protonation studies have been performed in 1,2-dichloroethane (DCE) solution with CF[subscript]3SO[subscript]3H at 25.0°C (eq 1). ML[subscript]n + CF[subscript]3SO[subscript]3H DCE [over] 25.0° C HML[subscript]n[superscript]+CF[subscript]3SO[subscript]3[superscript]-; [delta] H[subscript]HM\eqno(1);In the CpOs(PPh[subscript]3)[subscript]2X (X = I, Br, Cl, H) series, it was determined that the basicity ([delta]H[subscript] HM) of the osmium center increases in the order: I[superscript]- < Br[superscript]- < Cl[superscript]- << H[superscript]-. The hydride complex is 23 kcal/mol more basic than the iodide complex; in terms of an equilibrium constant, the hydride complex is 10[superscript]17 times more basic than the iodide complex. There is an excellent linear correlation between the basicities ([delta]H[subscript] HM) of the CpOs(PPh[subscript]3)[subscript]2X complexes and the gas phase proton affinities of the anions, X[superscript]-, toward the H[superscript]+. Changing the phosphine ligands in the CpOs(PR[subscript]3)[subscript]2Br complexes results in a linear increase in the basicity (-[delta]H[subscript] HM) from 16.3 kcal/mol to 29.4 kcal/mol in the order: PPh[subscript]3 < PPh[subscript]2Me < PPhMe[subscript]2 < PMe[subscript]3. Osmium complexes are 6.0-8.5 kcal/mol more basic ([delta]H[subscript] HM) than the analogous ruthenium complexes, and Cp* raises the basicity ([delta]H[subscript] HM) of a complex by 5.5-9.0 kcal/mol over the analogous Cp complexes;Studying multihydrido complexes ((H)[subscript]2Os(PR[subscript]3)[subscript]4, (H)[subscript]4Os(PR[subscript]3)[subscript]3, HIr(CO)(PPh[subscript]3)[subscript]3 and CpIr(ER[subscript]3)(H)[subscript]2] has shown that the basicity ([delta]H[subscript] HM) of the metal in (H)[subscript]4Os(PR[subscript]3)[subscript]3 (PR[subscript]3 = PPh[subscript]2Me, PPhMe[subscript]2) increases by ~15 kcal/mol when two hydride ligands are replaced by a PR[subscript]3 ligand to give (H)[subscript]2Os(PR[subscript]3)[subscript]4. Replacement of the two hydride ligands in CpIr(PPh[subscript]3)(H)[subscript]2 to give CpIr(PPh[subscript]3)(CO) results in a 10.4 kcal/mol increase in the basicity ([delta]H[subscript] HM) of the metal. Thus, comparison of the two electron donor ligand sets (H)[subscript]2, CO and PR[subscript]3 has illustrated that the basicity ([delta]H[subscript] HM) of the metals increase in the order: (H)[subscript]2 < CO < PPh[subscript]2Me < PPhMe[subscript]2. It has also been determined that the basicities of the CpIr(ER[subscript]3)(H)[subscript]2 complexes increase with ER[subscript]3 in the order: P(OPh)[subscript]3 << AsPh[subscript]3 ≈ PPh[subscript]3;In contrast to the above complexes protonation occurs at uncoordinated phosphorus atom of the [eta][superscript]1-dppm (Ph[subscript]2PCH[subscript]2PPh[subscript]2) ligand in M(CO)[subscript]5([eta][superscript]1-dppm) (M = Cr, Mo, W) and fac-M(CO)[subscript]3(N N)([eta][superscript]1-dppm) (M = Mo, N N = bipy, phen; M = W, N N = bipy). For dppm, its monoprotonated form (dppmH[superscript]+) and these complexes, the basicity (-[delta]H[subscript] HM) increases from 14.9 kcal/mol to 23.1 kcal/mol in the order: dppmH[superscript]+ < Cr(CO)[subscript]5([eta][superscript]1-dppm) < Mo(CO)[subscript]5([eta][superscript]1-dppm) < W(CO)[subscript]5([eta][superscript]1-dppm) < dppm ≤ fac-Mo(CO)[subscript]3([eta][superscript]2-bipy)([eta][superscript]1-dppm) < fac-Mo(CO)[subscript]3([eta][superscript]2-phen)([eta][superscript]1-dppm) ≈ W(CO)[subscript]3([eta][superscript]2-bipy)([eta][superscript]1-dppm). Thus, the H[superscript]+ is more electron with-drawing than M(CO)[subscript]5 (M = Cr, Mo, W) and M(CO)[subscript]3([eta][superscript]2-phen) and W(CO)[subscript]3([eta][superscript]2-bipy) actually enhance the basicity ([delta]H[subscript] HM) of the dangling phosphorus as compared to dppm itself.