A HF/CI-SD study of the low-lying states of nitroprusside ion

Since the discovering of two photoexcited metastable states of crystalline sodium nitroprusside, Na2Fe(CN)5NO.2H2O (SNP), showing rather long lifetimes at temperatures below 160 K, much effort has been devoted toward the study of its electronic structure. Despite this tremendous effort the nature of the frontier orbitals and the related low energy excitations remains controversial. Early calculations, EHT, showed the HOMO as mainly the metallic 3d orbital while the LUMO had a major p* (NO) contribution. However INDO calculations, clearly set the metal d orbital many electron-volts deep in core. The vertical electronic spectrum have been estimated through ab initio HF/CI-SD with a double-zetaF quality basis set. The ab initio results support Bottomley and Greins interpretations and assign the first electronic transitions to ligand-to-ligand charge-transfer excitation from trans-cyano to nitrosyl ligands. The corresponding oscillator strengths have been calculated showing comparable intensity with the experimental results. The excitation energy of the metal ® NO charge-transfer transition, 8e ® 13e (d xz,d yz ® p* NO) have been estimated to be at 4.52 eV and show a rather intense absorption band. The second CT excitation, 1b2 ® 13e (d xy ® p* NO), pointed by previous works as a typical CT band, exhibits a small intensity at 5.04 eV. In the calculations it was observed that SCF orbital ordering are rather dependent on the metal basis set used. Metallic minimal basis set show results in close agreement with EHT early calculations while double-zeta basis set pushes the metallic d orbitals deep away from the HOMOs. The HF orbital ordering has been used to interpret photochemical and thermoanalysis experiments on SNP and the results seem to fit properly with the calculated properties

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 approximation (GGA) level has been applied to the complexes [Fe(CN)5L]n- and [Ru(CN)5L]n- (L = pyridine, pyrazine, N-methylpyrazinium), as well as to [Fe(CN)5]3- and [Ru(CN)5]3-. Full geometry optimizations have been performed in all cases. The geometrical parameters are in good agreement with available information for related systems. The role of the MII-L back-bonding was investigated by means of a L and cyanide Mulliken population analysis. For both Fe(II) and Ru(II) complexes the metal-L dissociation energies follow the ordering pyridine < pyrazine < N-methyl pyrazinium, consistent with the predicted σ-donating and π*-accepting abilities of the L ligands. Also, the computed metal-L bond dissociation energies are systematically smaller in the Ru(II) than in the Fe(II) complexes. This fact suggests that previous interpretations of kinetic data, showing that ruthenium complexes in aqueous solution are more inert than their iron analogues, are not related to a stronger Ru-L bond but are probably due to solvation effects