Rhodium complexes four-coordinate

It is noteworthy that, in the carboxamidate-ligated dirhodium(II) complexes (86), (87), and (89), the rhodium core is coordinated by four ligands and two N and two O atoms are bound to each rhodium center, constituting a unique coordination sphere (Figure 10).225... 

The most thoroughly investigated homogeneous hydrogenation catalyst is the four-coordinate rhodium complex Rhf (C6H5)3P]3C1. This catalyst is called... 

Homogeneous hydrogenation catalyzed by the four-coordinated rhodium complex, Rh[(C6H5)3P]3Cl, has been particularly well investigated. With this catalyst, the first step is formation of the six-coordinated rhodium hydride of known configuration, 16, in which we abbreviate the ligand, triphenylphosphine, (C6H5)3P, as L ... 

The distortion is the result of the strain imposed on the whole molecule by the geometric requirements of the corrole structure. It is not, however, very significant the rhodium atom is displaced by only 0.26 A from the plane of the four coordinating nitrogen atoms, a value much smaller than that observed in the structure of the P-unsubstituted complex Co(Corrole)PPh3 shown in Fig. 12 where the cobalt atom is displaced by 0.38 A from the macrocycle plane [32]. In both compounds the four coordinating nitrogen atoms are strictly coplanar. 

These complexes can be prepared from phosphorus ligands that are themselves substituted by electron-withdrawing groups. It is often beneficial, however, to employ a low ratio of phosphorus ligand to rhodium since in several instances excess ligand cleaves the halo bridges and forms four-coordinate monomeric complexes. Table 2 lists those complexes of this structure that have been isolated. 

The carbethoxy tertiary phosphine Bu2P(CH2) C02Et (n — 1) forms an octahedral, cationic rhodium(III) complex (31), in which the carbonyl group coordinates to rhodium. Homologous ligands ( = 2, 3) from four-coordinate rhodium(II) complexes in which the ligands are only phosphorus bound.275... 

Note Abstraction of the bromide substituent creates a cationic, four coordinate and square planar rhodium(I) complex. 

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. 

Studies of for a series of square-planar complexes, pyramidal complexes [Rh(PPh3) (C0)X(S02)] showed the order of increasing frequencies to remain the same for the two series except for the nitrato complexes. This evidence was cited for Ti-interaction between the bonding rr-orbital of the nitrate group and the /)j orbital of rhodium in the four-coordinate complex. However, this is not possible in the five-coordinate complex since thep orbital participates in CT-bonding with the unshared electron pair of sulfur (241). 

Dichlorotetracarbonyldirhodium reacts readily with phosphines and arsines to form the four-coordinate phosphine or arsine carbonyl chlorides. - The complexes may also be prepared by refluxing rhodium (III) chloride 3-hydrate with a large excess of triphenylphosphine or triphenylarsine in 2-methoxyethanol. ... 

Thus the Berry coordinate represents a viable option for intramolecular exchange in rhodium and iridium complexes, in contrast to platinum and palladium complexes. Nickel complexes, on the other hand, can adopt either tetrahedral or square-planar conformations in the four-coordinate structures, and therefore the fact that these complexes can take on any of the three conformations is not surprising. This analysis is described in detail in Reference 67. 

It would be more interesting and useful if the reaction could be made catalytic. Actually, catalytic decarbonylation reaction was found to be possible by using chlorocarbonylbis(triphenylphosphine) rhodium (XII) (26). This complex is reasonably stable, and more importantly it is four-coordinated and coordinatedly unsaturated, so that it may expand to a six-coordinated complex by the oxidative addition of acyl halides or aldehydes. The oxidative addition of methyl iodide to similar complexes was reported by Heck (5). 

Organometallic compounds of rhodium have the metal center in oxidation states ranging from +4 to -3. but the most common oxidation states are +1 and +3. The Rh(I) species have a d electron configuration and both four coordinated square planar and five coordinated trigonal bipyramidal species exist. Oxidative addition reactions to Rh(I) form Rh(III) species with octahedral geometry. The oxidative addition is reversible in many cases, and this makes catalytic transformations of organic compounds possible. Presented here are important reactions of rhodium complexes in catalytic and stoichiometric transformations of organic compounds. 

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