Structural and electron density studies on a chromium(IV)-oxo complex [Cr-IV(O)(TMP)]

The electron density distribution of a chromium(IV)-oxo complex, [Cr-IV(O)( TMP)] (TMP = 5,10,15,20-tetrakis-p-methoxyphenyl porphyrin), is investigate d by molecular orbital calculation. The molecular and crystal structure of the compound is studied by x-ray diffraction. It belongs to the space group I2, Z = 2, a = 14.979(4) Angstrom, b = 9.752(3), c = 15.605(3) Angstrom, b eta = 100.97(2)degrees, V = 2238(1) Angstrom(3), MO K alpha radiation lambd a = 0.7107 Angstrom, R = 4.9%, Rw = 3.5% for 3575 observed reflections. Cr is five-coordinated in a square pyramidal fashion with the Cr atom located 0.42 Angstrom toward the oxo-ligand. Deformation density maps are derived f rom the single point molecular orbital calculation on the basis of HF and D FT(density functional theory) calculations. The accumulation of deformation density along the C-H, C-C, C-N and C-O bonds in the porphyrin ligand is w ell represented. The asphericity in electron density around the Cr ion is c learly demonstrated. Natural ;bond orbital analysis (NBO) reveals that the Cr-O-oxo is actually a triple-bond character (sigma(2)pi(4)) and the four N of pyrrole serves as a sigma-donor to Cr. The Cr-N-pyrole bond is essentia lly a dative bond. d-Orbital populations of Cr derived from both calculatio ns are in good agreement with each other. Planar d(pi)-orbital is the most populated, which is in accord with the prediction from crystal field theory . Detail bond characterization of the Cr-L multiple bond is discussed.