Rhodium tetravalent

In these the metal is divalent, tetravalent, and trivalent respectively. The ammino-iridous and the ammino-iridic salts correspond to the ammino-derivatives of palladium and platinum, whilst those of the sesqui-salts are analogous to the ammino-derivatives of cobalt, chromium, and rhodium. 

It is well known that chemical shifts for meso protons of the octaethylporphyrin complexes depend upon the oxidation state of the central metal. Divalent metals have a resonance in the region of 9.75-10.08 ppm while for tri- and tetravalent metals this resonance is in the range of 10.13-10.39 and 10.30-10.58 ppm respectively. For the In(OEP)(R) series the indium oxidation state is III and the observed methine proton shifts of 10.17-10.37 ppm correspond to those of a typical trivalent metal (Table 3). The exact position of the In(OEP)(R) methinic protons resonance varies systematically with the electron-donating ability of the axial ligand i.e. the more basic the axial ligand, the higher the field. Similar results are observed for o-bonded complexes of the rhodium series . ... 

Rhodium can occur as a cation in the monovalent, trivalent and tetravalent states. Although the latter of these is likely to be unstable in water, two studies by Kalinina etal. (1977) and Kalinina and Lyakushina (1977) have reported stability constants for the oxo-ion, RhO " ". There is no supporting evidence for either of the species proposed or the form in which rhodium(lV) exists, and, as such, the data are not accepted. A solubility constant for rhodium(l) oxide is available (van Muylder and Pourbaix, 1974), although it is not clear from where this datum was originally sourced. The reported solubility is log /Csio = 3.31, but no confirmatory evidence has been presented for this solubility. Rhodium(III) is the dominant aqueous form of the element, and some stability constant data are available for the first monomeric hydrolysis species RhOH " ". Data are not available for any other species. Shaimon (1976) has indicated that the ionic radius of Rh is 0.665 A. 

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