STRUCTURE AND BONDING IN PENTACYANO(L)FERRATE(II) AND PENTACYANO(L)RUTHENATE(II) COMPLEXES (L=PYRIDINE, PYRAZINE, AND N-METHYLPYRAZINIUM) -A DENSITY-FUNCTIONAL STUDY

Density Functional Theory (DFT) at the generalized gradient approximat ion (GGA) level has been applied to the complexes [Fe(CN)(5)L](n-) and [Ru(CN)(5)L](n-) (L = pyridine, pyrazine, N-methylpyrazinium), as wel l as to [Fe(CN)(5)](3-) and [Ru(CN)(5)](3-). Full geometry optimizatio ns have been performed in all cases. The geometrical parameters are in good agreement with available information for related systems. The ro le of the M(II)-L back-bonding was investigated by means of a L and cy anide Mulliken population analysis. For both Fe(II) and Ru(II) complex es the metal-L dissociation energies follow the ordering pyridine < py razine < N-methyl pyrazinium, consistent with the predicted sigma-dona ting and pi-accepting abilities of the L ligands. Also, the computed metal-L bond dissociation energies are systematically smaller in the R u(II) than in the Fe(II) complexes. This fact suggests that previous i nterpretations of kinetic data, showing that ruthenium complexes in aq ueous solution are more inert than their iron analogues, are not relat ed to a stronger Ru-L bond but me probably due to solvation effects.