sigma-bond activation by transition metal complexes and its analogous reactions. A theoretical study

Though C-H sigma -bond activation of methane by platinum(0) and palladium(0 ) complexes, M-(PH3)(2) (M=Pd or Pt), is significantly endothermic and requ ires a considerably large activation barrier, a platinum(0) chelate-phosphi ne complex can perform the C-H sigma -bond activation. This is interpreted in terms of distortion energy and d orbital energy. However, a palladium(0) complex cannot perform it even with a chelate-phosphine. An electron-withd rawing CN group on the sp(3) C atom also accelerates the C-H sigma -bond ac tivation, because the CN group stabilizes the transition state through a ch arge-transfer interaction between Pd d and CN pi* orbitals and strengthens the palladium-alkyl bond. As a result, the palladium(0) chelate-phsophine c omplex can perform the C-H sigma -bond activation of dicyanomethane. On the other hand, a palladium(II) carboxylate complex easily perfomrs the C-H si gma -bond activation of methane and benzene through heterolytic C-H bond fi ssion. This bond fission is assisted by conversion of the carboxylate ligan d to carboxylic acid.