The gas phase and solvent dependent preference of the tautomerization between 2-pyridinethiol (2SH) and 2-pyridinethione (2S) has been assessed using variable temperature Fourier transform infrared (FTIR) experiments, as well as ab initio and density functional theory computations. No spectroscopic evidence (nu(S)(-)(H) stretch) for 2SH was observed in toluene, C(6)D(6), heptane, or methylene chloride solutions. Although, C(s)() 2SH is 2.61 kcal/mol more stable than C(s)() 2S (CCSD(T)/cc-pVTZ//B3LYP/6-311+G(3df,2p)+ZPE), cyclohexane solvent-field relative energies (IPCM-MP2/6-311+G(3df,2p)) favor 2S by 1.96 kcal/mol. This is in accord with the FTIR observations and in quantitative agreement with the -2.6 kcal/mol solution (toluene or C(6)D(6)) calorimetric enthalpy for the 2S/2SH tautomerization favoring the thione. As the intramolecular transition state for the 2S, 2SH tautomerization (2TS) lies 25 (CBS-Q) to 30 kcal/mol (CCSD/cc-pVTZ) higher in energy than either tautomer, tautomerization probably occurs in the hydrogen bonded dimer. The B3LYP/6-311+G(3df,2p) optimized C(2) 2SH dimer is 10.23 kcal/mol + ZPE higher in energy than the C(2)(h)() 2S dimer and is only 2.95 kcal/mol + ZPE lower in energy than the C(2) 2TS dimer transition state. Dimerization equilibrium measurements (FTIR, C(6)D(6)) over the temperature range 22-63 degrees C agree: K(eq)(298) = 165 +/- 40 M(-)(1), DeltaH = -7.0 +/- 0.7 kcal/mol, and DeltaS = -13.4 +/- 3.0 cal/(mol deg). The difference between experimental and B3LYP/6-311+G(3df,2p) [-34.62 cal/(mol deg)] entropy changes is due to solvent effects. The B3LYP/6-311+G(3df,2p) nucleus independent chemical shifts (NICS) are -8.8 and -3.5 ppm 1 A above the 2SH and 2S ring centers, respectively, and the thiol is aromatic. Although the thione is not aromatic, it is stabilized by the thioamide resonance. In solvent, the large 2S dipole, 2-3 times greater than 2SH, favors the thione tautomer and, in conclusion, 2S is thermodynamically more stable than 2SH in solution.

Reaction of H-Cr(CO)(3)C(5)Me(5) with *NO at 1-2 atm pressure in toluene solution yields Cr(NO)(CO)(2)C(5)Me(5) as the sole metal-containing product in addition to N(2)O and HNO(2) as the principle nitrogen-containing products. N(2)O and HNO(2) are attributed to decomposition of the initial product HNO. Kinetic studies yield the rate law d[P]/dt = -k(2nd)( )(order)[HCr(CO)(3)C(5)Me(5)][*NO]; k(2nd)( )(order) = 0.14 M(-)(1) s(-)(1) at 10 degrees C, with DeltaH() = 11.7 +/- 1.5 kcal/mol and DeltaS() = -16.3 +/- 3.5 cal/(mol deg). The rate of reaction is not inhibited by CO. The kinetic isotope effect for reaction of D-Cr(CO)(3)C(5)Me(5) is k(H)/k(D) = 1.7. These observations are consistent with a first step involving direct H (D) atom transfer from the metal hydride to *NO, forming HNO. Also supporting this mechanism is the approximately 150-times slower reaction of H-Mo(CO)(3)C(5)Me(5) and failure to observe reaction for H-W(CO)(3)C(5)Me(5) in keeping with metal-hydrogen bond strengths Cr < Mo < W. Reaction of PhS-Cr(CO)(3)C(5)Me(5) with NO at 1-2 atm pressure in toluene solution also forms Cr(NO)(CO)(2)C(5)Me(5) as the sole metal-containing product. The initial product is the unstable nitrosothiol PhS-NO. Kinetic studies yield the rate law d[P]/dt = -k(1st)( )(order)[PhS-Cr(CO)(3)C(5)Me(5)]; k(1st)( )(order) = 3.1 +/- 0.3 x 10(-)(3) s(-)(1) at 10 degrees C, with DeltaH() = 21.6 +/- 1.2 kcal/mol, DeltaS() = + 3.9 +/- 1.5 cal/(mol deg). The rate of reaction is independent of both NO and CO pressure. The transition state in the first-order process is proposed to involve migration of bound thiolate to coordinated CO, forming Cr(CO)(2) (eta(2)-C(=O)SPh)C(5)Me(5). The enthalpy of reaction of *Cr(CO)(3)C(5)Me(5) and NO yielding Cr(NO)(CO)(2)C(5)Me(5) and CO has been measured by solution calorimetry: DeltaH degrees = -33.2 +/- 1.8 kcal/mol. The Cr-NO bond strength is estimated as approximately 70 kcal/mol and provides the net thermodynamic driving force for the proposed elimination of the unstable molecules HNO and PhSNO.

Enthalpies of oxidative addition of PhE-EPh (E = S, Se, Te) to the M(0) complexes M(PiPr3)2(CO)3 (M = Mo, W) to form stable complexes M(*EPh)(PiPr3)2(CO)3 are reported and compared to analogous data for addition to the Mo(III) complexes Mo(N[tBu]Ar)3 (Ar = 3,5-C6H3Me2) to form diamagnetic Mo(IV) phenyl chalcogenide complexes Mo(N[tBu]Ar)3(EPh). Reactions are increasingly exothermic based on metal complex, Mo(PiPr3)2(CO)3 < W(PiPr3)2(CO)3 < Mo(N[tBu]Ar)3, and in terms of chalcogenide, PhTe-TePh < PhSe-SePh < PhS-SPh. These data are used to calculate LnM-EPh bond strengths, which are used to estimate the energetics of production of a free *EPh radical when a dichalcogenide interacts with a specific metal complex. To test these data, reactions of Mo(N[tBu]Ar)3 and Mo(PiPr3)2(CO)3 with PhSe-SePh were studied by stopped-flow kinetics. First- and second-order dependence on metal ion concentration was determined for these two complexes, respectively, in keeping with predictions based on thermochemical data. ESR data are reported for the full set of bound chalcogenyl radical complexes (PhE*)M(PiPr3)2(CO)3; g values increase on going from S to Se, to Te, and from Mo to W. Calculations of electron densities of the SOMO show increasing electron density on the chalcogen atom on going from S to Se to Te. The crystal structure of W(*TePh)(PiPr3)2(CO)3 is reported.