THEORETICAL-STUDIES OF INORGANIC AND ORGANOMETALLIC REACTION-MECHANISMS .2. THE TRANS EFFECT IN SQUARE-PLANAR PLATINUM(II) AND RHODIUM(I) SUBSTITUTION-REACTIONS

Ab initio calculations with an effective core potential have been used to study the mechanism of substitution reactions for square-planar transition-metal complexes. Pseudo-trigonal-bipyramidal transition states with rather small entering-ligand to metal to leaving-ligand angles (70-85-degrees) were found for the substitution reactions investigated in this paper. The stability of the transition state is determined by both sigma and pi effects of the ligands. The analysis of the Laplacian of the total electron density indicated that there are two electron concentrations located in the equatorial plane of the pseudo-trigonal-bipyramidal transition state. These are the same concentrations that one finds above and below the plane of the square-planar d8 reactant. For a strong pi-acceptor as the trans-directing ligand, these two concentrations are stabilized through pi-back-donation. A strong sigma-donor as the trans-directing ligand weakens the bonds between metal and entering/leaving ligands in the transition state and therefore decreases the electron repulsion between each of the two lone pairs on the central metal and the lone pairs on the entering/leaving ligands. The trans effect is thus rationalized by the sigma and pi effects on the stability of the transition state.