TRIDENTATE LIGAND EFFECTS ON ENTHALPIES OF PROTONATION OF (L-3)M(CO)(3) COMPLEXES (M = W, MO)

Titration calorimetry has been used to determine the enthalpies of pro tonation (Delta H-HM) for the reaction of (L-3)M- (CO)(3) complexes, w here 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 (-Del ta H-HM) increase with the ligand donor groups (X, Y, or Z) in the ord er 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 protona ted product. Although the metal basicity (-Delta H-HM) generally incre ases as the ligand donor group basicities (pK(a)'s of the conjugate ac ids) increase, the large difference between the pK(a) values of thioet hers (-6.8) and phosphines (6.25) suggests that thioether donor groups should be much weaker donors than phosphines. The observation that th ioether groups contribute nearly as much as phosphine groups to the ba sicity 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 th e d orbitals thereby making the metal more basic than expected from on ly 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 analo gous Mo complexes is linear (r = 0.9996), and the slope of 1.07 indica tes that the tridentate ligands have nearly the same electronic effect s on both W and Mo complexes. Noncyclic ligands make the metal more ba sic by 1.6 +/- 0.3 kcal/mol than cyclic ligands with the same donor at oms. The tungsten complexes are 2.8 +/- 0.1 kcal/mol more basic than t heir 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 f or both the tungsten (-2.0 kcal/mol) and molybdenum (-4.8 kcal/mol) co mplexes. These studies demonstrate that the metal, ligands, and geomet ry of the protonated products all substantially affect the heats of pr otonation (Delta H-HM) Of (L-3)M(CO)(3) complexes.