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Using the QALE model, we determined the electronic parameters for PF3 (χd = 44 ± 4, E ar = 0, πp = 14 ± 1), PCl3 (χd = 42 ± 1, E ar = 4.1 ± 0.3, πp = 5.3 ± 0.5), PH3 (χd = 17 ± 1, E ar = 0, πp = 3.7 ± 0.7), and P(CH2CH2CN)3 (χd = 17.0 ± 0.6, E ar = 0, πp = 1.2 ± 0.2). These values indicate that PF3 and PCl3 are comparable in σ donor ability and are the poorest σ donor ligands we have studied. Both PH3 and P(CH2CH2CN)3 are reasonably good σ donors, comparable in strength to P(p-ClC6H4)3. PF3 is by far the best π acid. The π acidity of PCl3 is comparable to that of P[(OCH2)3]CEt, whereas the π acidity of PH3 is intermediate between P(O-p-XC6H4)3 and P(OR)3. The analysis of data sets containing PZ3 - i H i sometimes requires the inclusion of “i” as a parameter, which we connect with changes in hybridization of these ligands. There are good correlations of χd and πp with the theoretical results of Gonzalez-Blanco and Branchadell and those of Fantucci. The stereoelectronic parameters for 107 PZ3 species are listed.

Comparison of pairs of physiochemical properties of phosphines and their complexes demonstrates that, in general, at least four independent stereoelectronic parameters are in general required to describe the variations in these properties. Consequently, any two-parameter model such as Drago's E/C model will have limited use in correlation analyses involving phosphines. The QALE analysis of E B and C B shows that these parameters are linear combinations of the QALE parameters χ and θ, with a small contribution from E ar. Thus, successful application of the E/C model will be restricted to the situation where the property being analyzed depends primarily on χ and θ.

1H NMR spectra for solutions prepared by dissolution of [CpCr(CO)3]2 and [Cp*Cr(CO)3]2 in toluene in the temperature range 190−390 K are interpreted in terms of thermodynamic and kinetic parameters for dissociation of the diamagnetic dimers into the paramagnetic monomers CpCr(CO)3 and Cp*Cr(CO)3. There is no evidence in this temperature range for thermally populated excited states or non-Curie magnetic behavior of the monomers making a significant contribution to the NMR. An expression for the temperature dependence of the NMR chemical shift at limiting fast interchange of monomer and dimer in terms of the ΔH° and ΔS° for dimer dissociation is applied in determining the thermodynamic parameters for Cr−Cr bond homolysis of [CpCr(CO)3]2 ( = 15.3 ± 0.6 kcal mol-1; = 39 ± 2 cal K-1 mol-1) and [Cp* Cr(CO)3]2 ( = 14.2 ± 0.4 kcal mol-1; = 47 ± 2 cal K-1 mol-1). Rate constants and activation parameters have been evaluated from 1H NMR line broadening in the region of slow dimer−monomer interchange for dissociation of [CpCr(CO)3]2 (k 1 (240 K) ≈ 59 s-1; = 17 ± 2 kcal mol-1; = 21 ± 6 cal K-1 mol-1) and [Cp*Cr(CO)3]2 (k 2 (240K) ≈ 1.4 × 104 s-1; = 16 ± 1 kcal mol-1; = 30 ± 6 cal K-1 mol-1). Paramagnetic shifts also were used in deriving electron−proton coupling constants (A H) for CpCr(CO)3 (8.22 × 105 Hz) and Cp*Cr(CO)3 (1.33 × 106 Hz).

Two methods are described and illustrated for the measurement of organo−cobalt bond homolysis energies through reactions of tetra(p-anisyl)porphyrinato cobalt(II), (TAP)CoII•, with organic radicals of the form •C(CH3)(R)CN in the presence of olefins. Thermodynamic values for bond homolysis have been determined directly for (TAP)Co-C(CH3)2CN (ΔH° = 17.8±0.5 kcal mol-1, ΔS° = 23.1 ± 1.0 cal K-1 mol-1) and (TAP)Co-CH(CH3)C6H5 (ΔH° = 19.5 ± 0.6 kcal mol-1, ΔS° = 24.5 ± 1.1 cal K-1 mol-1) from evaluation of the equilibrium constants for the dissociation process (Co−R ⇌ CoII• + R•) in chloroform. The bond homolysis enthalpy for (TAP)Co-C5H9 (ΔH° = 30.9 kcal mol-1) was determined indirectly by measuring the thermodynamic values for the competition reaction (TAP)Co-C(CH3)2CN + C5H8 ⇌ (TAP)Co-C5H9 + CH2C(CH3)CN (ΔH° = 0.9 ± 0.3 kcal mol-1) in conjunction with a thermochemical cycle. This indirect approach was also used to evaluate (TAP)Co-CH(CH3)C6H5 BDE (20.5 kcal mol-1) which agrees favorably with the value determined directly. When the Co−R bond homolysis enthalpies are known from independent evaluation, these equilibrium measurements provide a method for evaluating relative heats of formation of organic radicals. Application of this approach gives 40.8 kcal mol-1 for the heat of formation of •C(CH3)2CN in chloroform. Success of these methods is dependent on fast abstraction of H• from the organic radicals by (TAP)CoII• to form (TAP)Co-H and rapid addition of (TAP)Co-H with olefins to form organocobalt complexes. Kinetic-equilibrium simulations utilizing reaction schemes for these processes provide an accurate description of the kinetic profiles and the equilibrium concentrations of solution species when the organic radical species achieve steady state.

Cobalt(II) porphyrin complexes ((por)Co II •) are used to illustrate how metalloradicals (Μ•) can function to control radical polymerization through both chain transfer catalysis and living polymerization. Chain transfer catalysis (CTC) is best achieved when there are minimal steric demands. This allows β-hydrogen abstraction from oligomer radicals by Μ•, as illustrated by the radical polymerization of methyl methacrylate in the presence of tetraanisylporphyrinato cobalt(II). When β-Η abstraction from the oligomer radical is precluded by sterics, then a metalloradical mediated living radical polymerization (LRP) can occur. Radical polymerization initiated and mediated by organo-cobalt tetramesitylporphyrin complexes manifest high living character as shown by the linear increase in M n with conversion, formation of block copolymers and relativity low polydispersity homo and block copolymers. Kinetic studies provide rate and activation parameters for the living radical polymerization process.Bond homolysis of an organometallic complex (M-C(CH 3 )(R)X) in solution proceeds through the intermediacy of a caged radical pair (M e »C(CH 3 )(R)X) that can recombine, separate into freely diffusing radicals, or react by Μ· abstracting a β-Η from the organic radical to form a metal hydride (M-H) and an olefin ( 7).In the absence of events that irreversibly terminate radicals and metal hydride, the homolytic dissociation of an organo-metal complex can potentially provide a constant equilibrium source of both an organic radical and a metal hydride. The broad objectives of this program are to evaluate the kinetic and thermodynamic factors that

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