Hydride ligand, five coordinate

There are now a number of quite stable Pt(IV) alkyl hydride complexes known and the synthesis and characterization of many of these complexes were covered in a 2001 review on platinum(IV) hydride chemistry (69). These six-coordinate Pt(IV) complexes have one feature in common a ligand set wherein none of the ligands can easily dissociate from the metal. Thus it would appear that prevention of access to a five-coordinate Pt(IV) species contributes to the stability of Pt(IV) alkyl hydrides. The availability of Pt(IV) alkyl hydrides has recently allowed detailed studies of C-H reductive elimination from Pt(IV) to be carried out. These studies, as described below, also provide important insight into the mechanism of oxidative addition of C-H bonds to Pt(II). 

The question of which pathway is preferred was very recently addressed for several diimine-chelated platinum complexes (93). It was convincingly shown for dimethyl complexes chelated by a variety of diimines that the metal is the kinetic site of protonation. In the system under investigation, acetonitrile was used as the trapping ligand L (see Fig. 1) which reacted with the methane complex B to form the elimination product C and also reacted with the five-coordinate alkyl hydride species D to form the stable six-coordinate complex E (93). An increase in the concentration of acetonitrile led to increased yields of the methyl (hydrido)platinum(IV) complex E relative to the platinum(II) product C. It was concluded that the equilibration between the species D and B and the irreversible and associative1 reactions of these species with acetonitrile occur at comparable rates such that the kinetic product of the protonation is more efficiently trapped at higher acetonitrile concentrations. Thus, in these systems protonation occurs preferentially at platinum and, by the principle of microscopic reversibility, this indicates that C-H activation with these systems occurs preferentially via oxidative addition (93). 

Both five-coordinate and four-coordinate pathways have been proposed for these reactions. The associative (five-coordinate) mechanism involves the formation of a trigonal bipyramidal or square pyramidal intermediate, which can revert back to tetracoordination by alkene insertion into the Pt—H bond.151 The dissociative (four-coordinate) mechanism involves initial substitution of a ligand other than hydride by alkene, followed by insertion to form the alkyl product. The ligand which is substituted is usually the anionic ligand, and if this group is trans to hydride an isomerization will need to occur prior to insertion of the coordinated alkene into the Pt—H bond. 

Hydrogenation proceeds by hydride rearrangement of 17 to a five-coordinated ethyl-rhodium complex, 18. This complex regains a ligand molecule to replace the one lost previously, thereby giving the six-coordinated complex, 19 ... 

Fig. 14. HOs5C(CO),j(OP(OCHj)2)(P(OCHj)j), 14 (34). The Os,C core is similar to that in Fig. 13. In this case Os(4) and Os(5) occupy apical sites and Os( I), Os(2), and Os(3) are three of the equatorial sites of the incomplete pentagonal bipyramid. The trimethylphosphite ligand is coordinated to Os(3), which also bears two carbonyls and the phosphonate oxygen atom. The hydride is assigned a bridging position on Os(l)-Os(2). The two metal-metal bonds to Os(3) are significantly longer [2.954(20) A] than the remaining five [mean 2.877(9) A]. The mean Os—Cpjrtjjdj distance is 2.07(7) A.

The electrochemical reduction of a series of cationic iron(II) hydrido complexes trans-[FeH(L)(dppe)2]+ (L = N2, C5H5N, PhCN, CH CHCN, MeCN, P(OMe)3, P(OEt)3, CO) has been shown to proceed via transient d1 iron(I) species which lose one of the neutral ligands. Further reduction of the resulting five-coordinate intermediates yields stable rf8 iron(O) anionic hydrides.233... 

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