Titration calorimetry has been used to determine the enthalpies of protonation (Delta H-HM) for the reaction of (L-3)M- (CO)(3) complexes, where M = W and Mo and L-3 = cyclic and noncyclic tridentate ligands of the types CH2- CH2-X-CH2CH2-Y-CH2CH2-Z and RX-CH2CH2-Y-CH2CH2-XR with N, S, and P donor atoms, with CF3SO3H in 1,2-dichloroethane solution at 25 degrees C to give (L-3)M(CO)(3)(H)+CF3SO3-. The basicities (-Delta H-HM) increase with the ligand donor groups (X, Y, or Z) in the order S less than or equal to PPh << NR (R = Me, Et) for both cyclic and noncyclic ligand complexes that have the same structure of the protonated product. Although the metal basicity (-Delta H-HM) generally increases as the ligand donor group basicities (pK(a)'s of the conjugate acids) increase, the large difference between the pK(a) values of thioethers (-6.8) and phosphines (6.25) suggests that thioether donor groups should be much weaker donors than phosphines. The observation that thioether groups contribute nearly as much as phosphine groups to the basicity of the metal in the (L-3)M(CO)(3) complexes may be explained by suggesting that repulsion between the pi-symmetry lone electron pair on sulfur and the filled metal d orbitals increases the energies of the d orbitals thereby making the metal more basic than expected from only the a-donor ability of the sulfur. There is a good correlation (r = 0.973) between -Delta H-HM and average nu(CO) values of the eight (L-3)W(CO)(3) complexes that have the same structure of their protonated forms. A plot of the average of the three nu(CO) frequencies for the (L-3)W(CO)(3) complexes vs the average nu(CO) frequencies for the analogous Mo complexes is linear (r = 0.9996), and the slope of 1.07 indicates that the tridentate ligands have nearly the same electronic effects on both W and Mo complexes. Noncyclic ligands make the metal more basic by 1.6 +/- 0.3 kcal/mol than cyclic ligands with the same donor atoms. The tungsten complexes are 2.8 +/- 0.1 kcal/mol more basic than their molybdenum analogs. Determinations of Delta H-HM values for both fac- and mer-(PNP)M(CO)(3) complexes (M = W, Mo; PNP = MeN(C2H4PPh2)(2)) allowed the calculation of enthalpies of mer-to-fac isomerization for both the tungsten (-2.0 kcal/mol) and molybdenum (-4.8 kcal/mol) complexes. These studies demonstrate that the metal, ligands, and geometry of the protonated products all substantially affect the heats of protonation (Delta H-HM) Of (L-3)M(CO)(3) complexes.