The enthalpies of reaction of neat PBu 3 with solid sulfur (-27.1 ( 0.5 kcal/mol), selenium (-20.0 ( 0.6 kcal/ mol), and tellurium (-4.9 ( 0.6 kcal/mol) have been measured by solution calorimetry. The enthalpies of reaction of a series of phosphines with sulfur in toluene solution have been measured as follows (values in kcal/mol): PCy 3 ) -30.9 ( 1.9, PBu 3 ) -28.9 ( 0.3, PMe 3 ) -27.1 ( 0.4, PMe 2 Ph ) -26.0 ( 0.5, PMePh 2 ) -23.8 ( 0.3, PPh 3 ) -21.5 ( 0.3. These values correlate with literature data for enthalpies of protonation and indicate that P to S σ donation is probably the dominant factor in determining the R 3 PdS bond strength, estimates for which range from 88 to 98 kcal/mol. The enthalpies of S atom transfer to PPh 3 by SdAsPPh 3 and SdSbPh 3 in toluene solution are -17.7 ( 1.2 and -21.5 ( 1.0 kcal/mol, respectively. The enthalpy of removal of the central S atom from BzSSSBz by PCy 3 , yielding BzSSBz and SdPCy 3 , is -29.0 ( 1.8 kcal/mol. These data are used to establish a range of enthalpies of S atom transfer in these compounds which spans 31 kcal/mol from SdSbPPh 3 to SdPCy 3 .

Vibrational data (IR, Raman and inelastic neutron scattering) and a supporting normal coordinate analysis for the complex trans-W(CO)3(PCy3)2(η2-H2) (1) and its HD and D2 isotopomers are reported. The vibrational data and force constants support the well-established η2-bonding mode for the H2 ligand and provide unambiguous assignments for all metal−hydrogen stretching and bending frequencies. The force constant for the HH stretch, 1.3 mdyn/Å, is less than one-fourth the value in free H2 and is similar to that for the WH stretch, indicating that weakening of the H−H bond and formation of W−H bonds are well along the reaction coordinate to oxidative addition. The equilibrium isotope effect (EIE) for the reversible binding of dihydrogen (H2) and dideuterium (D2) to 1 and 1-d 2 has been calculated from measured vibrational frequencies for 1 and 1-d 2. The calculated EIE is “inverse” (1-d 2 binds D2 better than 1 binds H2), with K H/K D = 0.78 at 300 K. The EIE calculated from vibrational frequencies may be resolved into a large normal mass and moment of inertia factor (MMI = 5.77), an inverse vibrational excitation factor (EXC= 0.67), and an inverse zero-point energy factor (ZPE = 0.20), where EIE = MMI × EXC × ZPE. An analysis of the zero-point energy components of the EIE shows that the large decrease in the HH stretching frequency (force constant) predicts a large normal EIE but that zero-point energies from five new vibrational modes (which originate from translational and rotational degrees of freedom from hydrogen) offset the change in zero-point energy from the H2(D2) stretch. The calculated EIE is compared to experimental data obtained for the binding of H2 or D2 to Cr(CO)3(PCy3)2 over the temperature range 12−36 °C in THF solution. For the binding of H2 ΔH = −6.8 ± 0.5 kcal mol-1 and ΔS = −24.7 ± 2.0 cal mol-1 deg-1; for D2 ΔH = −8.6 ± 0.5 kcal/mol and ΔS = −30.0 ± 2.0 cal/(mol deg). The EIE at 22 °C has a value of K H/K D = 0.65 ± 0.15. Comparison of the equilibrium constants for displacement of N2 by H2 or D2 in the complex W(CO)3(PCy3)2(N2) in THF yielded a value of K H/K D = 0.70 ± 0.15 at 22 °C.

Reaction of Mo(N[R]Ar)(3) (R = (t)Bu or C(CD(3))(2)CH(3)) with N(2)O gives rise exclusively to a 1:1 mixture of nitride NMo(N[R]Ar)(3) and nitrosyl ONMo(N[R]Ar)(3), rather than the known oxo complex OMo(N[R]Ar)(3) and dinitrogen. Solution calorimetry measurements were used to determine the heat of reaction of Mo(N[R]Ar)(3) with N(2)O and, independently, the heat of reaction of Mo(N[R]Ar)(3) with NO. Derived from the latter measurements is an estimate (155.3 +/- 3.3 kcal.mol(-1)) of the molybdenum-nitrogen bond dissociation enthalpy for the terminal nitrido complex, NMo(N[R]Ar)(3). Comparison of the new calorimetry data with those obtained previously for oxo transfer to Mo(N[R]Ar)(3) shows that the nitrous oxide N-N bond cleavage reaction is under kinetic control. Stopped-flow kinetic measurements revealed the reaction to be first order in both Mo(N[R]Ar)(3) and N(2)O, consistent with a mechanism featuring post-rate-determining dinuclear N-N bond scission, but also consistent with cleavage of the N-N bond at a single metal center in a mechanism requiring the intermediacy of nitric oxide. The new 2-adamantyl-substituted molybdenum complex Mo(N[2-Ad]Ar)(3) was synthesized and found also to split N(2)O, resulting in a 1:1 mixture of nitrosyl and nitride products; the reaction exhibited first-order kinetics and was found to be ca. 6 times slower than that for the tert-butyl-substituted derivative. Discussed in conjunction with studies of the 2-adamantyl derivative Mo(N[2-Ad]Ar)(3) is the role of ligand-imposed steric constraints on small-molecule, e.g. N(2) and N(2)O, activation reactivity. Bradley's chromium complex Cr(N(i)Pr(2))(3) was found to be competitive with Mo(N[R]Ar)(3) for NO binding, while on its own exhibiting no reaction with N(2)O. Competition experiments permitted determination of ratios of second-order rate constants for NO binding by the two molybdenum complexes and the chromium complex. Analysis of the product mixtures resulting from carrying out the N(2)O cleavage reactions with Cr(N(i)Pr(2))(3) present as an in situ NO scavenger rules out as dominant any mechanism involving the intermediacy of NO. Simplest and consistent with all the available data is a post-rate-determining bimetallic N-N scission process. Kinetic funneling of the reaction as indicated is taken to be governed by the properties of nitrous oxide as a ligand, coupled with the azophilic nature of three-coordinate molybdenum(III) complexes.