Electrochemical Parametrization in Sandwich Complexes of the First Row Transition Metals

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 protonation (DeltaH(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 N, S, and P donor atoms, with CF(3)SO(3)H in 1,2-dichloroethane solution at 25 degrees C to give (L(3))M(CO)(3)(H)(+)CF(3)SO(3)(-). The basicities (-DeltaH(HM)) increase with the ligand donor groups (X, Y, or Z) in the order S </= 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 (-DeltaH(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 sigma-donor ability of the sulfur. There is a good correlation (r = 0.973) between -DeltaH(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 DeltaH(HM) values for both fac- and mer-(PNP)M(CO)(3) complexes (M = W, Mo; PNP = MeN(C(2)H(4)PPh(2))(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 (DeltaH(HM)) of (L(3))M(CO)(3) complexes.

Modeling of the Redox Properties of (Hexaamine)cobalt(III/II) Couples

The thermodynamics (redox potentials) and kinetics (electron transfer rates) of (hexaamine)cobalt(III/II) redox couples are interpreted in terms of steric strain induced by the ligand systems. The intersections of potential energy curves (strain energy versus metal-ligand distance plots of pairs of conformers) of the oxidized and reduced forms of a wide range of (hexaamine)cobalt(III/II) couples are related to the inner sphere reorganization (DeltaH()), and correlated with experimentally determined electron self-exchange rates. The minima of these potential energy curves of the reduced and oxidized forms are correlated with the reduction potentials. The perturbation by electronic effects due to differences in nucleophilicity along the series ammonia, primary amine, secondary amine, tertiary amine has been accounted for. The redox potentials of the couples studied (E degrees = -0.6V to +0.8 V; vs SHE), the electron self-exchange rates (10(-)(7)s(-)(1)-10(3)s(-)(1)), the Co(3+)-N distances (1.94-2.05 Å), and the ligand field strengths (Co(3+): (1)A(1) -->( 1)T(1), 16 700-22 200 cm(-)(1)) cover a wide range. Accurate computed values for extremely long Co(3+)-N bonds and for the corresponding low ligand field parameters (MM-AOM), high redox potentials, and specific electron self-exchange rates could only be obtained with a modification of the originally used force field, involving Morse potentials for the metal-ligand bonds. Applications of these methods, involving the design of new oxidants or reductants with specific potentials and electron transfer rates, and the determination of solution structures based on experimentally determined redox properties are presented, limits of this purely steric approach are discussed, and alternatives are evaluated.