Rhodium—chlorine bonds

The interesting complex chemistry of rhodium has been rather neglected this is probably because most of the synthetic methods for obtaining complexes have been tedious. In general, substitutions of chlorine atoms bonded to rhodium by other ligands are slow, and products have usually been mixtures. The situation is now changing, since novel catalytic approaches to rhodium complexes have been developed.1 The catalysis in the present synthesis involves the rapid further reaction of the hydrido complex formed from l,2,6-trichIorotri(pyridine)rho-dium(III) in the presence of hypophosphite ion. 

The substantially larger values of M-p for phosphorus trans to chlorine compared with phosphorus trans to phosphorus correlate with shorter M-P bonds trans to chlorine for a number of metals and oxidation states tungsten(IV), rhodium(I) and (III), platinum(II) and (IV), and linear mercury(II) (15). By analogy with the discussion of the results for the platinum(II) complexes, this indicates the dominance of the (P sMSp)2 term in Equation 1 for couplings with a variety of M, but as discussed earlier it is difficult to determine the extent of variation of... 

RhCl(PPh3) 3 The chlorine radical (Cl ) accepts an electron from rhodium metal (electronic configuration Ad1,5s2) to give Cl and Rh+. The chloride ion then donates two electrons to the rhodium ion to form a dative or a coordinate bond. Each PPh3 donates a lone pair of electrons on the phosphorus atom to the rhodium ion. The total number of electrons around rhodium is therefore 8 + 2 + 3X2=16, and the oxidation state of rhodium is obviously 1 +. The other way of counting is to take the nine electrons of rhodium and add one electron for the chlorine radical and six for the three neutral phosphine ligands. This also gives the same electron count of 16. 

It is well known that the chlorine-bridged dimeric complex (which does not contain a metal bond) obtained from Wilkinson s catalyst can react via stepwise addition of H2 to rhodium (equation 84) (see Rhodium Organometallic Chemistry). 

The dimer (XXVI) yields the FeCla complex (XXVII) containing four coordinate rhodium, the fourth chlorine atom bridging rhodium and iron. XXVI also reacts with o-phenanthroline, pyridine, and carbon monoxide to give complexes of the type (TPPO)RhCl L or (TPPO)RhCl in which TPPO remains ly -bonded to the metal. 

Two different strategies have been developed for C-Cl activation with rhodium compounds (a) oxidative addition of C-Cl bonds to electron-rich Rh(I) complexes and (b) -coordination of coordinatively unsaturated, electron-deficient Rh(III) species with the benzene ring of ArCI, followed by aromatic nucleophilic substitution of chlorine in the thus activated aromatic system. 

A proposed mechanism [9] for the hydrosilylation of olefins catalyzed by platinum(II) complexes (chloroplatinic acid is thought to be reduced to a plati-num(II) species in the early stages of the catalytic reaction) is similar to that for the rhodium(I) complex-catalyzed hydrogenation of olefins, which was advanced mostly by Wilkinson and his co-workers [10]. Besides the Speier s catalyst, it has been shown that tertiary phosphine complexes of nickel [11], palladium [12], platinum [13], and rhodium [14] are also effective as catalysts, and homogeneous catalysis by these Group VIII transition metal complexes is our present concern. In addition, as we will see later, hydrosilanes with chlorine, alkyl or aryl substituents on silicon show their characteristic reactivities in the metal complex-catalyzed hydrosilylation. Therefore, it seems appropriate to summarize here briefly recent advances in elucidation of the catalysis by metal complexes, including activation of silicon-hydrogen bonds. 

The order of stabilities of the adducts was the same as that observed previously for additions of the hydrosilanes to complexes [14, 32c, 18], i.e., negative substituents such as alkoxy groups or chlorine atoms on silicon stabilize the adducts. Furthermore, rate measurements have indicated that the structure of hydrosilanes does not affect markedly the rate ( i) of the forward reaction (oxidative addition), but affects strongly the rate ( i) of the reverse reaction (reductive elimination). This latter fact, in addition to a possible dependence of the stability of metal-silicon bonds on metal species (c.g., rhodium vs, platinum) will be reflected in the catalysis by particular metal complexes, which is clearly shown in the following sections. Another approach has been to study the stereochemistry of an optically active... 

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