Square planar complexes rhodium-olefin

In order to model the square planar rhodium(I) complex we need to realize that the positions trans to the diphosphine may not be equivalent, since the diphosphine is chiral. Consider the [(diphosphine)Rh(norbornadiene)] as a model for the solvento species. In order to distinguish between the nonequivalent phosphorus atoms, we label them and P ,. Each olefin is 90° from one phosphorus atom... 

Typical square-planar rhodium-olefin complexes such as acetylacetonates (48) have a stoichiometry of two coordinated olefins per metal-atom. Since chelating olefins are bidentate in their cationic rhodium biphosphine complexes, it would be surprising if bis-olefin complexes were never found under hydrogenation conditions. It seems clear, in fact, that they can be the major coordinated species under certain conditions. Thus examples of 2 1 rhodium enamide complexes with biz-diphenyl-phosphinopropane have been observed (49), although the majority of cases involve a8-unsaturated acids co-complexed with DIOP. 

Dissociative mechanisms for square-planar substitutions are discussed in a review. A molecular orbital study of insertion of ethene into Pt—H bonds concludes that the reaction can be best described by a series of, preferably, dissociative steps. Rearrangements of three-co-ordinate ML3 T- or Y-shaped i -structures are discussed in this context. Three-co-ordinate intermediates are also suggested in the mechanisms for palladium(ii)-catalysed oxidations of olefins, and for electrophilic cleavage of platinum-carbon ff-bonds by protons. Parallel associative and dissociative processes have been proposed for a substitution reaction of a square-planar rhodium(i) complex in benzene solution. Especially, sterically crowded complexes have been thought to stabilize three-co-ordinate intermediates more easily. Recently determined activation volumes for sterically hindered square-planar complexes both of platinumand palladium are not compatible with dissociative activation, however. 

A simplified mechanism for the hydroformylation reaction using the rhodium complex starts by the addition of the olefin to the catalyst (A) to form complex (B). The latter rearranges, probably through a four-centered intermediate, to the alkyl complex (C). A carbon monoxide insertion gives the square-planar complex (D). Successive H2 and CO addition produces the original catalyst and the product ... 

As in other 6e complexes, the rhodium atom in [Rh2Cl2(C2H4)4] has square-planar coordination with olefin C = C double bonds perpendicular to the... 

The hydrosilylation catalysis either by dimeric or by monomeric rhodium and iridium siloxide complexes occurs via preliminary oxidative addition of hydrosilane, followed by elimination of disiloxane (detected by gas chromatography-mass spectrometry) to generate square-planar 16e complex containing M—H and TT-bonded olefins (for catalysis by monomeric Rh and Ir complexes of vinylsilane hydrosilylation, see (Scheme 10)) (82). 

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