Platinum dissociative substitution

Dissociative substitution at square-planar centers (231), particularly platinum(II), was a matter of controversy for several years, with the... 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

The steric crowding introduced in the latter by the four ethyl substituents inhibits nucleophilic attack at platinum, so that complexes of this type tend to undergo substitution by a dissociative mechanism [89]. The complex of the more rigid ligand, 2,2, 2"-terpyridyl, Pt(terpy)Cl+, is found to be about 103 to 104 times more reactive to substitution than the dien analogue this is ascribed to steric strain [90], which is reflected in the short Pt—N bond to the central nitrogen (Pt-N some 0.03 A shorter than the other two Pt-N bonds) and N—Pt—N bond angles of 80-82°). 

The compounds of zero valent platinum Pt(Ph3P)4 and Pt(Ph3P)3 were first discovered in 1958. Tetrahedral complexes Pt(Pp3)4 and Pt(P(OEt)3)4 undergo nucleophilic substitution by a dissociative mechanism (Sec. 4.8). 

A useful method to probe whether the reaction mechanism involves an associative or dissociative pathway is to measure AV (the volume of activation) for the reaction. High pressure kinetics in methanol give AV 1 —12 cm3 mol-1 for an associative first step, and +7.7 cm3 mol"1 for the isomerization reaction. It is proposed that the faster reaction is a solvolytic replacement of Cl" followed by a dissociative isomerization step with [PtR(MeOH)(PEt3)2]+ (R = alkyl, aryl equation 210).580 Since isomerization and substitution reactions are mechanistically intertwined, it is useful to note here that for the rates of substitution of both cis- and frara,-PtBr(2,4,6-Me3C6H2)(PEt3)2 by I" and thiourea, the volumes of activation are negative, in support of associative processes.581 Further support for associative solvation as the first step in the isomerization of aryl platinum(II) complexes has been presented,582 and the arguments in favor summarized.583... 

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