Abstract: Semi-empirical molecular orbital methods within the framework of the finite perturbation theory, the FPT-CNDO/INDO methods, which consist of the FPT-CNDO/2 and FPT-INDO methods, are set up for the study of chemical shielding in transition metal compounds, and a corresponding computational program is developed on VAX 11/785 computer to establish a theoretical study of the transition metal chemical shielding by quantum chemistry methods. Application of the methods has been carried out in the calculation of ⁹⁵Mo chemical shielding constants of mononuclear precursors[MoO_nS_4-n]^2-(n=0-4). With the standard CNDO/INDO parameters a linear regression was obtained between the calculated results and the corresponding experimental data:δ_cal=(0.8345 δ_exp-43.83)ppm, with a correlation coefficient of 0.999. Investigation on the calculated electronic configuration confirms that in[MoO_nS_4-n]^2-(n=0-4)⁹⁵Mo chemical shifts are dominated by the d-orbital paramagnetic contribution arising from the d-d transition. Applications were also carried out in the calculation of ⁹⁵Mo chemical shielding constants in Cu-Mo-S clusters. Sensitivity of the method towards CNDO/INDO parameters is surveyed and study is focused on the d-orbital exponent of Mo atom. Adjusted parameters obtained by a trial and error procedure produce results of ⁹⁵Mo chemical shielding constants which agree with the corresponding experimental data:σ_cal¹=(-1.104 δ_exp-1026.8)ppm, with a correlation coefficient of-0.97. Application of the FPT-CNDO/INDO methods is extended to the study of the ⁵⁹Co chemical shielding constants in hexa-coordinated diamagnetic Co(Ⅲ) complexes. Tha calculated values give good agreement with experimental chemical shifts. The discrepancy between the calculated and the experimental chemical shifts can also be attributed to the radial factor effect.
The following conclusions can be made:1. for transition metal elements ⁹⁵Mo and ⁵⁹Co, the chemical shielding constants are dominated by the d-orbital paramagnetic contributions; 2. electron delocalization oceur in the Cu-Mo-S clusters, and diffuse d-orbitals of Mo and Cu atoms are required for the description of the electron distribution in the Cu-Mo-S clusters; 3. among the CNDO/INDO parameters, d-orbital exponents of the transition metal atoms, operating through the radial factor -3>, are the most sensitive ones, and results of modest accuracy can be obtained as long as little adjustment is made to the CNDO/INDO parameters.