Rhodium chloro complex

Our study on the synthesis, structure and catalytic properties of rhodium and iridium dimeric and monomeric siloxide complexes has indicated that these complexes can be very useful as catalysts and precursors of catalysts of various reactions involving olefins, in particular hydrosilylation [9], silylative couphng [10], silyl carbonylation [11] and hydroformylation [12]. Especially, rhodium siloxide complexes appeared to be much more effective than the respective chloro complexes in the hydrosilylation of various olefins such as 1-hexene [9a], (poly)vinylsiloxanes [9b] and allyl alkyl ethers [9c]. 

Noble metals, such as gold, platinum, palladium, rhodium and iridium are not attacked by nitric acid at ordinary temperatures. These metals, however, dissolve in aqua regia (3 1 HCl— HNOs mixture). Nitric acid in aqua regia oxidizes gold to Au , which readily combines with Cl to form soluble chloro-complex, AuClF. 

Chloro-bridged dimeric rhodium(I) complexes, such as [Rh(CO)2Cl]2 (27, 57, 98) and [Rh(COD)Cl]a (25), react with polymeric resins to give monomeric polymer-bound complexes with phosphine and amine supports. 

Chlorotris(triphenylphosphine)rhodium(I) is also the precursor of numerous chloro complexes which may be obtained by exchanging PPh3 for other ligands. The main use of this reaction, however, is to prepare [RhXL2L ] complexes (see Section 48.4.3.1 below). The triphenylphosphine substitution reactions of [RhCl(PPh3)3] are shown in Scheme 6. 

Bis(3-diphenylphosphinopropyl)phenylphosphine (99) forms chloro, bromo and iodo rhodium(I) complexes (equations 253 and 254). The bromo complex can be obtained by addition of a large excess of LiBr to [RhCl(cod)]2 prior to addition of the neutral ligand.1051 The iodo complex has also been prepared from [RhI(CgH12)]2.1051,1052 The halo complexes (100) have been shown to be square planar by X-ray crystallography.1051... 

Two related orange brown /i-hydrido-/i-chloro complexes (141) can also be obtained from [IrH5(PEt3)2]. Further structural details are available. The 31P NMR spectrum of (141) (PPh3 = PEt3) has a doublet of doublets at 80p.p.m. and a set of four triplets at 58.8p.p.m. The latter group arises from the phosphorus atom bound to rhodium which is cis to the hydrido bridge.1348... 

Siloxy-rhodium(I) complexes of general formula [(diene)Rh(//-OSiMe3)]2, where diene = cod, nbd, showed much higher catalytic activity in the hydrosilylation of 1-alkenes [50] and allyl ethers [51] by triethoxy silane than the respective chloro-rhodium(I) complexes [(diene)RhCw-Cl)2] suggesting a possible application of dimeric bridged siloxy-metal complexes as potent precursors of a variety of hydrosilylation reactions. 

The hydrogenation activity which was very low for the Tt-chloro-bridged neutral rhodium(I) complexes 3 could be enhanced tremendously by reaction with silver tetrafluoroborate according to Fig. 3, transforming them into cationic species 4 possessing two additional free coordination sites to bind both the substrates, olefin and hydrogen, in the transition state during the catalytic reaction. 

There arc two main classes of catalysts neutral chlororhodium(I) diphosphane complexes and cationic rhodium(I) complexes having the general structure [Rh(diphosphane)(olefin)2]+. The cationic complexes are often more active and more selective than the corresponding chloro complexes60. In contrast to heterogeneous hydrogenation catalysts rhodium phosphane complexes are not pyrophoric. However, they are sensitive to oxygen and should be stored and handled under an inert atmosphere. 

In this Section, we confine our attention to the complexes where the non-equivalence of the phosphorus atoms derives from the stereochemistry of the complex, rather than from the groups attached to phosphorus within the ligand. The results for some chloro-complexes are given in Table III. The meridianal form of the rhodium(III) complexes has the structure (12), which is of the same symmetry as the... 

Rhodium was discovered in 1804 by the Enghsh chemist Wilham Wollaston. Its name derives from the Greek word rhodos, meaning rose—the color of solutions containing rhodium salts. The abtmdance of rhodium in Earth s crust is approximately 0.0004 ppm. Its purification requires its separation from other platinum metals. Treatment of a crude platinum with metal concentrate with aqua regia leaves an insoluble portion that is then fused with bisulfate this dissolves only the rhodium component, which is eventually converted to a solution of chloro complexes, whose treatment with H2 precipitates the metal. 

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