Structural and Electron Density Studies on a Chromium(IV)-Oxo Complex [Cr~IV(O)(TMP)]

摘要

The electron density distribution of a chromium(IV)-oxo complex, [Cr~IV(O)(TMP)] (TMP = 5, 10, 15, 20-tetrakis-p-methoxyphenyl porphyrin), is investigated 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) A, b = 9.752(3), c = 15.605(3) A, #beta# = 100.97(2) deg C, V = 2238(1) A~3, Mo K#alpha# radiation #lambda# = 0.7107 A, R = 4.9 percent, Rw = 3.5 percent for 3575 observed reflections. Cr is five-coordinated in a square pyramidal fashion with the Cr atom located 0.42 A toward the oxo-ligand. Deformation density maps are derived from the single point molecular orbital calculation on the basis of HF and DFT (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 well represented. The asphericity in electron density around the Cr ion is clearly 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_pyrrole bond is essentially a dative bond. d-Orbital populations of Cr derived from both calculations 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.