Rhodium complexes tertiary arsine

Rhodium(Il) complexes with tertiary arsines were erroneously reported over 40 years ago. These complexes were, with the advent of NMR spectrometry, later proved to be hydridorhodium(III) complexes. Nevertheless, the only stable isolable monomeric rhodium(II) complexes are those containing tertiary phosphine and other similar group VB ligands. 

There are no tertiary arsine complexes of rhodium(II). Early reports of such complexes are erroneous.5,6 The complexes have been shown to be hydridorhodium(III) species. 

There is an extensive chemistry of tertiary phosphine rhodium(III) complexes. However, there are comparatively few complexes of monodentate tertiary arsines, although the complexes of ditertiary arsines are more numerous. There are virtually no tertiary stibine complexes. The two main preparative routes to the complexes described in this section are (i) direct reaction pf the ligands with rhodium trichloride, which usually yields trichloro complexes and (ii) oxidative addition to rhodium(I) tertiary phosphine complexes, which gives rise to more diverse products of the type [RhXYZ(PRj) ], Metathetical reactions on the complexes prepared by either method (i) or (ii) have been used to prepare most of the remaining compounds. 

Apart from the triethylarsine972 and the tribenzylarsine974 complexes the remaining tertiary arsine complex is formed by the ditertiary arsine l,2-(Ph2As)2C2H4 clearly this cannot adopt structure (80). However, the ditertiary stibine complexes976 may well adopt structure (80), since their stoichiometry implies that only half the antimony atoms are coordinated to rhodium. 

The tertiary arsine complexes were first prepared by Dwyer and Nyholm,5 but, before the advent of NMR spectrometry, were erroneously considered to be rhodium(II) complexes.989 Further investigations have shown that in preparations starting from rhodium trihalides using a higher tertiary arsine rhodium ratio and lower reaction temperatures favor the formation of the p isomer (equations 205 and 206).986... 

The tertiary arsine complexes (Table 74) are much less numerous than those containing tertiary phosphines. They are usually prepared by the interaction of tertiary arsines with rhodium trihalides. The poorer reducing properties of tertiary arsines make it much less likely that rhodium(I) complexes will be formed in this reaction. 

However, over the last 60 years a new type of chemistry has emerged. Although the first examples, tertiary arsine complexes, were initially prepared as an extension of classical rhodium(III) chemistry, the newer complexes containing tt-bonding ligands have been a consequence of the intense interest in the catalytic properties of rhodium(I) complexes. Examples of these ligands also include tertiary phosphines and stibines, although it is debatable to what extent they act as r-acids when coordinated to rhodium(III). 

The tridentate ligands (38)-(40) form rhodium(I) complexes. The complexes of the first two ligands readily undergo oxidative addition to form rhodium(III) complexes. The complex [RhCl(38)] also adds either SO2 or BF3 to form pentacoordinate rhodium(I) complexes. The tetraden-tate ligands (41) (Z = P, As) and (42) and the hexadentate ligand (43) form both rhodium(I) and (III) complexes. By contrast, the tri(tertiary arsine) ligand (44) fails to reduce hydrated rhodium trichloride and forms both fac- and mer-trihalorhodium(in) complexes. 

By contrast tertiary arsine and stibine ligands do not displace alkadiene ligands and are only able to displace weakly bound alkene - or allyl " ligands from other rhodium(I) complexes. 

Although trialkyl- and triarylbismuthines are much weaker donors than the corresponding phosphoms, arsenic, and antimony compounds, they have nevertheless been employed to a considerable extent as ligands in transition metal complexes. The metals coordinated to the bismuth in these complexes include chromium (72—77), cobalt (78,79), iridium (80), iron (77,81,82), manganese (83,84), molybdenum (72,75—77,85—89), nickel (75,79,90,91), niobium (92), rhodium (93,94), silver (95—97), tungsten (72,75—77,87,89), uranium (98), and vanadium (99). The coordination compounds formed from tertiary bismuthines are less stable than those formed from tertiary phosphines, arsines, or stibines. 

Rhodium(III) forms a wide range of complexes with tertiary phosphines and arsines [108, 109], though in some cases other oxidation states are possible. Table 2.5 summarizes the complexes produced from reaction of RhCl3 with stoichiometric quantities of the phosphine. 

To some extent this situation has been rectified by a number of reviews of more limited range. Among these are the review by Robinson on the rhodium(II) carboxylates,19 and the reviews on the chemistry of [RhCl(PPh3)3]20 and [RhH(CO)(PPhj)3].21 The tertiary phosphine, arsine and stibine complexes of the element have also been covered in two reviews of these ligands complexes with the transition elements.22... 

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