Coordination of Monodentate Ligands

Mechanistic studies of C-C reductive elimination from six-coordinate platinum(lV) complexes have consistently indicated the involvement of five-coordinate species on the reaction pathway (see Sect. 2.1.1). If the novel five-coordinate Pt(lV) complexes that have been isolated over the last decade are to serve as functional models for these intermediates, then direct C-C coupling, without preUminaiy Ugand association or dissociation, should be observable from the alkyl complexes. Studies of the thermolyses of several five-coordinate Pt(lV)Me3 complexes have provided convincing evidence for direct, concerted C-C reductive efiminatirHi from the five-coordinate species. 

Ethane, the expected product of C-C reductive elimination was observed upon 

Five-coordinate Pt(TV) species with silyl ligands are poised to perform Si-C or Si-H reductive elimination from Pt(IV). Note that the microscopic reverse of the latter reaction, Si-H oxidative addition, was used to synthesize the first five-coordinate Pt(TV) complexes with silyl ligands (2a-c) [84]. Complex 6, which has Pt-Me groups and a Pt-SiMe3 group, was observed to react over time at room temperamre to form tetramethylsilane, the product of Si-C reductive elimination, and intractable Pt products [91]. The five-coordinate complex ( pypyr)Pt(H 2SiEt3, 7a, was found to react with HSiMeaEt to form product 7b. Study of this reaction showed that Si-H reductive eliminaticm from 7a was rate-determining and it occurred directly from the five-coordinate complex [97]. Reaction of 7a with phosphines at room temperature led to the formation of a Pt(II)H(PR3) complex and free silane, the product of Si-H reductive elimination. Complex 7a was observed to be an active catalyst for the hydrosilylation of ethylene, tert-butylethylene, and alkynes [97]. 

The syntheses of isolable five-coordinate Pt(IV) complexes have allowed the direct study of C-C and Si-H reductive elimination processes. Rate constants can now be determined for these bond-forming reactions that do not include contributions from preliminary dissociation of ligands as is generally the case when the reactions occur from octahedral complexes. In additimi, reductive elimination from five-coordinate Pt(IV) complexes allows access to highly reactive Pt(II) species, which have been shown to perform intra- and intermolecular C-H activation and to provide entry points into catalytic hydroarylation and hydrosilylation reactions. 

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