Correlation between structure and spectral characteristics of rhodium(I) chelate dicarbonyl complexes and their electron

Abstract

Theoretical Hartree-Fock, MP2 and density functional (SVWN, BLYP, B3LYP) computations have been first made to calculate the equilibrium geometry and vibration frequencies of CO rhodium(I) chelate dicarbonyl complexes with monocharged bidentate ligands like β-diketonate R1COCHCOR2, iminoketonate R1COCHC(NH)R2, and β-diiminate R1C(NH)CHC(NH)R2 with substituents at carbon atoms R1, R2 = H, CH3, CF3. Consideration of electron correlation was shown to be especially important for the reasonable representation of bond distances with metal atom and CO vibration frequencies. The MP2 computation of exchange-correlation effects, as compared to density functional technique, results in a better description of frequency difference of symmetrical and asymmetrical modes of CO vibrations: inaccuracy in Δν(CO) description recedes from 12–16 cm−1 to 4 cm−1. The dependence between electron density distribution in complexes and spectral characteristics of terminal carbonyl groups was traced. Alteration of CO vibration frequencies was interpreted in terms of electron density shift from occupied d-orbitals of the metal to π*-orbitals of carbonyl groups. Population of CO π *-orbitals increases and CO vibration frequencies decrease on lessening of the electronegativity of donor atoms of a chelate ligand when passing on from oxygen to nitrogen and further among the substituents R CF3>H>CH3.