Rhodium trihalides

Rhodium trihalides (and complexes like K3RhBr6) are frequently added to photographic emulsions in trace quantities to improve the gradation of the emulsion (the rhodium effect ) [15]. 

A series of sulphide complexes can be made by refluxing the rhodium trihalide with the appropriate organic sulphide in methanol or ethanol ... 

The tertiary phosphine Ph2P(CH2COOEt) has been shown by Braunstein and his co-workers to form a rhodium(III) complex when allowed to react with rhodium trihalides. The products are the facial isomers (77). This is in accordance with their NMR spectra which show V(P-P) = 23.5 Hz.956... 

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. 

The phosphine combines with cobalt(II) dihalides to give the 5-coordinate derivatives, LXXVIII, but these do not undergo ionization in organic solvents. Rhodium (III) trihalides give normal 6-coordinate compounds, LXXIX, whereas sodium hexachloroiridate(IV), Na2lrCl6,... 

Rhodium is the whitest metal in the platinum group metals and it does not lose its luster under any atmospheres at ordinary temperatures. When rhodium is heated, the protective coating of rhodium oxide forms at 700°C. Further, at llOO C the oxide is dissociated to rhodium and oxygen. Rhodium dissolves in hot sulfuric acid or in aqua regia. Rhodium reacts with a halogen at high temperatures to afford a trihalide [1-6]. Rhodium has a high reflexibility and is used as a reflex mirror. The alloy with rhodium is used in thermocouples, crucibles, catalysts, electrical contact points, heat-resistant materials, corrosion resistant materials, etc. 

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