Square-planar substitution reactions trigonal bipyramids

Fig. 13.3 Reaction coordinate/energy profile for a square planar substitution reaction having (a) a trigonal bipyramidal activated complex and (b) a trigonal bipyramidal intermediate. [From Burdetl, 1. K. Inorg. Chem. 1977, 16, 3013-3025. Used with permission.)...

Nickel(II) phosphine complexes have been reported to he efficient catalysts in carbonylation reactions. To investigate this reaction mechanism, we have studied the reaction of CO on the related Ni(II) complexes NiX2(PMes)n (n = 2,3) and [NiX(PMes)m]BFj> (m = 3,4). Pentacoordinate carbonyl nickel(II) species (without reduction of Ni(II) to Ni(0)) were isolated (1) by direct substitution of PMcs by CO in the pentacoordinate complex and (2) by addition of CO on the trans square-planar tetracoordinate complex. These compounds are trigonal-bipyramidal complexes with CO in equatorial position. The Ni-CO distance (1.73 A) is the shortest reported Ni-CO distance. Since these carbonylation reactions can be viewed as substitution of an equatorial PMes by CO in a TBP, they can be related to the substitution reactions in square-planar d metal complexes. 

These carbonylation reactions can be viewed as substitution of an equatorial ligand PMe3 by CO in a d trigonal bipyramid and thus can be related to the well-known substitution reaction in planar (P transition metal complexes (13), the trigonal bipyramid being the ground state and the square planar the transition state or intermediate. 

The ligand substitution reactions of square planar complexes of Pt(II) of the type PtA2XL are stereospecific. If the substrate is a cis-isomer, the product is invariably a cis-isomer and vice versa. The reaction occurs via trigonal bipyramidal intermediate, as shown in Figure 10. 

Good examples of the associative mechanism are substitution reactions in complexes with a square planar geometry, which proceed via a trigonal bipyramid geometry, as shown in Figure 11.4. 

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