Square planar complexes kinetic stability

Substitution Reactions in Square Planar Complexes 538 Thermodynamic and Kinetic Stability 547 Kinetics of Octahedral Substitution 548 Mechanisms of Redox Reactions 557... 

The d metal ions, such as Pt(II), Pd(II) and Ni(II), often fonai square planar complexes. The square planar complexes of Pt(II) are of particular interest in kinetic studies due to their high stability, ease of synthesis and moderate rates of reaction that enable the monitoring of the reaction. The area of discussion in these complexes is restricted only to the substitution reactions. As compared to the octahedral complexes, the crowding around the metal ion is less in square planar complexes. This is one of the important reasons that most of the substitution reactions in these complexes follow the SN (associative mechanism). 

The kinetics and mechanism of ligand substitution reactions of square-planar platinum(II) dimethyl sulfoxide complexes have been exhaustively studied (173), and these workers conclude that the cis and trans influences and the trans effects of Me2SO and ethylene are similar in magnitude whereas the cis effect of Me2SO is about 100 times as large as that of ethylene. The results for reaction (5), where the stability constants, Kt, are reported to be 1.5 x 108 (L = S-Me2SO) and 4.5 x 108 (L = ethylene) corroborate this analogy (213). 

The decatrienyl-Ni complex is seen to be preferably generated through ethylene insertion into the q -allyl-Ni" bond of 2 via a square-planar transition state TS[2-5] that occurs at a distance of 1.8-1.9 A for the emerging C-C o-bond (Fig. 6). This leads to the [Ni"(q q A,-decatrienyl)j species 5 as the kinetic insertion product, which represents the thermodynamically favorable form of the decatrienyl-Ni" configurations 5-7. Furthermore, additional ethylene or butadiene monomers are indicated to not accelerate the insertion via coordinative stabilization of any of the involved key species and are therefore not likely to assist the decatrienyl-Ni formation along 2- 5. 

If the transition state does involve this sort of interaction then the simple considerations based on solvent transfer activity coefficients are invalidated. The mechanism of substitution in square planar platinum(II) complexes, and in particular the role of the fifth and sixth positions in relation to reactant and transition state stability, is one of the most interesting and challenging mechanistic problems in transition-metal substitution kinetics, and there is no doubt that a systematic application of solvent activity coefficients to a range of neutral and charged reactant complexes will lead to a better insight into these problems. 

Pearson and Sweigart have reported one of the first kinetic studies involving substitution at square-planar nickel(n), in Ni"-dithiolate complexes. They discuss nucleophilic reactivity, leaving group, and trans effects, and report that the reactions follow an associative pathway, as expected. The solvent path is not important and, although for Pt" the formation of a five-co-ordinate intermediate may be rate-determining, for Ni" this step is fast compared with subsequent steps. The formation constant of the five-co-ordinate intermediate does, however, markedly affect the overall rates, and it seems that the trans effect for nickel(ii) operates through the stability of this intermediate. 

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