Theoretical studies of inorganic and organometallic reaction mechanisms. 2. The trans effect in square-planar platinum(II) and rhodium(I) substitution reactions
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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°) were found for the substitution reactions investigated in this paper. The stability of the transition state is determined by both σ and π effects of the ligands. The analysis of the Laplacian of the total electron density indicated that there arc 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 π-acceptor as the trans-directing ligand, these two concentrations are stabilized through π-back-donation. A strong σ-donor as the transdirecting 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 σ and π effects on the stability of the transition state. © 1991, American Chemical Society. All rights reserved.
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