Basic organometallic complexes are important in many catalytic and stoichiometric reactions. Thus, to determine the factors controlling metal basicity, the heats of protonation ([delta]H[subscript] HM) of organometallic complexes have been studied by titration calorimetry. The protonation reactions are done with triflic acid (CF[subscript]3SO[subscript]3H) in 1,2-dichloroethane (DCE) solution at 25.0°C (eq 1). ML[subscript]x+CF[subscript]3SO[subscript]3H DCE[over] 25.0° C [HML[subscript]x] [superscript]+CF[subscript]3SO[subscript]3[superscript]-; [delta] H[subscript]HM\eqno(1);Increasing the number of methyl groups (N[subscript] Me) in the cyclopentadienyl ring of Cp[superscript]'Ir(1,5-COD) (Cp[superscript]' = C[subscript]5Me[subscript]x H[subscript]5-x, x = 0, 1, 3-5) complexes results in an increase in Ir basicity by -5.7 kcal mol[superscript]-1 on going from ( C[subscript]5 H[subscript]5)Ir(1,5-COD) (-22.8 kcal mol[superscript]-1) to ( C[subscript]5 Me[subscript]5)Ir(1,5-COD) (-28.5 kcal mol[superscript]-1). When [delta]H[subscript] HM of Cp[superscript]'Ir(1,5-COD) is plotted against N[subscript] Me, a linear correlation is obtained with each Me group worth 1.1 kcal mol[superscript]-1 towards an increase in the basicity of Cp[superscript]'Ir(1,5-COD). An analysis of the effect of C[subscript]5 H[subscript]5 vs C[subscript]5 Me[subscript]5 on the basicity of Ir in terms of [delta]H[subscript] HM, [delta]G°, and [delta]S° is also discussed;Changing the phosphine ligand in CpIr(CO)(PR[subscript]3) complexes results in a linear increase in the basicity ([delta]H[subscript] HM) of Ir from -29.2 kcal mol[superscript]-1 (PR[subscript]3 = P(p-ClC[subscript]6H[subscript]4)[subscript]3) to -33.2 kcal mol[superscript]-1 (PR[subscript]3 = PMe[subscript]3). There is a linear correlation between [delta]H[subscript] HM and free phosphine basicity ([delta]H[subscript] HP) with a slope of 0.298. Furthermore, the basicities of Fe(CO)[subscript]3(PR[subscript]3)[subscript]2 complexes also correlate linearly with [delta]H[subscript] HP but a larger slope (0.916) is obtained. The basicities of the Ir and Fe also show linear correlations with their respective [upsilon](CO) values;The [delta]H[subscript] HM values of Fe(CO)[subscript]3[Ph[subscript]2 P(CH[subscript]2)[subscript]nPPh[subscript]2] (n = 1-4) complexes become less exothermic from -24.0 kcal mol[superscript]-1 (n = 1) to -20.1 kcal mol[superscript]-1 (n = 4) by increasing the size of the chelate. In contrast, the free bidentate phosphine ligands become more basic, as measured by [delta]H[subscript] HP, with increasing chelate size. The Fe chelate complexes are also substantially more basic than the respective non-chelate derivatives. The trend in Fe basicities with chelate size and the comparison to the basicities of the non-chelated compounds are ascribed to distortions of the Fe(CO)[subscript]3(L[superscript]\capL) complexes by the chelate ligand.