Rhodium with halides

The pattern of iridium halides resembles rhodium, with the higher oxidation states only represented by fluorides. The instability of iridium(IV) halides, compared with stable complexes IrCl4L2 and the ions IrX (X = Cl, Br, I), though unexpected, finds parallels with other metals, such as plutonium. Preparations of the halides include [19]... 

One can set up to do this using the competition between dimerization and halogen atom abstraction from RX to form the rhodium(III) halide complex. As a function of [RX], the product ratio is quite easily evaluated. From that, one can get the rate constant ratio but, knowing independently the rate constant for dimerization, it is possible to extract from those data rate constants for reactions of the rhodium(II) complex with these organic halides. The rate constants obtained are listed in Table I. 

Rhodium phthalocyaninates - Well-defined rhodium(III) phthalocyanines RhX(PcXMeOH) (X = Cl, Br, I) with halide and methanol as axial ligands were prepared for photochemical investigations [118]. 

Many papers formulate the starting chlororhodium(III) porphyrin just as RhCl(P), as if a trans ligand L in MC1(P)L were easily lost (path c, X = Cl). However, the conditions of preparations point to a predominance of hexacoor-dinate aqua species, RhCI(P)H20. Only in one case the formation of a pentacoordinated rhodium(III) halide, the iodide RhI(TMP), seems well-documented [61], see Sect. 2.1.3. The formation of interesting heterobimetallic porphyrins, e.g. (TPP)RhMn(CO)s, [path c, X = Mn(CO)s] was formulated as starting from RhCl(TPP) [63], but the work referred to [264] clearly stated that hexacoordinate species, namely RhCl(TPP)H20, RhCl(TPP)(EtOH), or RhCl(TPP)CO were involved. On the other hand, the heterobimetallic species appear to be pentacoordinate about the rhodium, in accord with many metal-metal-bonded porphyrin complexes [222] (see also below). 

Rhodium(III) halides form [RhX2 Sb(o-C6H4Me)3 ] complexes when allowed to react with tri(o-tolyl)stibine. Despite their stoichiometry and diamagnetism there is no evidence for their being either o-metallated or hydridorhodium(III) complexes. It is believed they have the planar dimeric structure (32). Intramolecular interactions in such dimers could account for their diamagnetism and the non-equivalence of the methyl protons in their H NMR spectrum.278... 

Complexes of this type can be prepared in three principal ways. The most obvious is by allowing hydrated rhodium trichloride to react with the bidentate ligand (equation 231).229 Unfortunately this method gives rise to both cis and trans products. However, the reactions between rhodium(III) halides and l,2-bis(diphenylphosphino)benzene (90) yield the trans product in the case of the chloride, whilst both the bromide and iodide form the cis product.1037... 

Rhodium and iridium carbonyl halides are also prepared from organic compounds, for example, anhydrous HCOOH and DMF, which decompose with carbon monoxide evolution. The following rhodium carbonyl halides are known [Rh2X2(CO)4], [RhX2(CO)2] - (X = Cl, Br, I), [Rh2X4(CO)2] (X = Br, I), [RhX3(CO)],... 

Rhodium.—The formation of rhodium(i) complexes with allenes has been much studied for rhodium(i)-halide compounds. Allenes also react with the diketone complexes Rh(LL)(C2H4)a where LL = acetylacetonato or dibenzoylmethanato, to form rhodium(i)-allene compounds. In this case an A"-ray crystal structure determination has shown that the product contains the allene tetramer (19) bonded to the rhodium by two w-allyl bonds. ... 

Rh(NH3)5X] +, where X = Cl, Br, or I, are also included in Table 13. The suggested mechanism involves slow dissociation of an adduct [Rh(NH3)s-XHg] + which is in equilibrium with the starting reactants. Both Hg +- and HgCl+-catalysed aquations of rhodium(ni)-halide complexes can be incorporated in the general kinetic versus stability-constant correlation for metalion catalysis of aquation of halogeno-complexes mentioned in Section 1 of this chapter. Silver(i) is not an efficient catalyst for the aquation of [Rh(NH3)5-Q]2+ 240 Redox catalysis of substitution at rhodium(m) has been reviewed. ... 

There is also clear evidence of a change from predominantly class-a to class-b metal charactristics (p. 909) in passing down this group. Whereas cobalt(III) forms few complexes with the heavier donor atoms of Groups 15 and 16, rhodium(III), and more especially iridium (III), coordinate readily with P-, As- and S-donor ligands. Compounds with Se- and even Te- are also known. Thus infrared. X-ray and nmr studies show that, in complexes such as [Co(NH3)4(NCS)2]" ", the NCS acts as an A -donor ligand, whereas in [M(SCN)6] (M = Rh, Ir) it is an 5-donor. Likewise in the hexahalogeno complex anions, [MX ] ", cobalt forms only that with fluoride, whereas rhodium forms them with all the halides except iodide, and iridium forms them with all except fluoride. 

CVD Reactions. The rhodium halides, like those of the other platinum group metal s, are volatile with a decomposition pointtoo close to the vaporization point to make them usable for CVD transport. The metal is commonly produced by the decomposition of metallo-organic precur-... 

Rhodium catalysts have also been used. Benzylic halides were converted to carboxylic esters with CO in the presence of a rhodium complex. In this case, the R could come from an ether R20, a borate ester B(OR )3, or an Al, Ti, or Zr alkoxide. Reaction with an a,co-diiodide, BU4NF and Mo(CO)e gave the corresponding lactone. ... 

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