Thermodynamic and activation parameters for dissociation of [CpCr(CO)(3)](2) and [Cp*Cr(CO)(3)](2) into paramagnetic monomers from H-1 NMR shift and line width measurements

H-1 NMR spectra for solutions prepared by dissolution of [CpCr(CO)(3)](2) a nd [Cp*Cr(CO)(3)](2) in toluene in the temperature range 190-390 K are inte rpreted in terms of thermodynamic and kinetic parameters for dissociation o f 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 pop ulated excited states or non-Curie magnetic behavior of the monomers making a significant contribution to the NMR. An expression for the temperature d ependence of the NMR chemical shift at limiting fast interchange of monomer and dimer in terms of the Delta H degrees and Delta S degrees for dimer di ssociation is applied in determining the thermodynamic parameters for Cr-Cr bond homolysis of [CpCr(CO)(3)](2) (Delta H(1)degrees = 15.3 +/- 0.6 kcal mol(-1); Delta S(1)degrees = 39 +/- 2 cal K-1 mol(-1)) and [Cp* Cr(CO)(3)]( 2) (Delta H(2)degrees = 14.2 +/- 0.4 kcal mol(-1); Delta S(2)degrees = 47 /- 2 cal K-1 mol(-1)). Rate constants and activation parameters have been e valuated from H-1 NMR line broadening in the region of slow dimer-monomer i nterchange for dissociation of [CpCr(CO)(3)](2) (k(1) (240 K) approximate t o 59 s(-1); Delta H(1)double dagger = 17 +/- 2 kcal mol(-1); Delta S(1)doub le dagger = 21 +/- 6 cal K-1 mol(-1)) and [Cp*Cr(CO)(3)](2) (k(2) (240K) ap proximate to 1.4 x 10(4) s(-1); Delta H(2)double dagger = 16 +/- 1 kcal mol (-1); Delta S(2)double dagger = 30 +/- 6 cal K-1 mol(-1)): Paramagnetic shi fts also were used in deriving electron-proton coupling constants (A(H)) fo r CpCr(CO)(3) (8.22 x 10(5) Hz) and Cp*Cr(CO)(3) (1.33 x 10(6) Hz).