and CrF . The + state for Mn is not represented in simple halides but MnO F is known, and beyond Mn no metal has a trihalide except FeX and CoF . The ability of fluorine to stabilise the highest oxidation state is due to either higher lattice energy as in the case of CoF , or higher bond enthalpy terms for the higher covalent compounds, e.g., VF and CrF . Although V + is represented only by VF , the other halides, however, undergo hydrolysis to give oxohalides, VOX .
Another feature of fluorides is their instability in the low oxidation states e.g., VX (X = CI, Br or I) . . Trends in the M + /M Standard Electrode Potentials . .
Trends in Stability of Higher Oxidation States + CrF + VF CrF + TiX VX I CrX MnF + TiX VX CrX MnF FeX I CoF + TiX III VX CrX MnX FeX CoX NiX CuX II ZnX + CuX III Oxidation Number Table . : Formulas of Halides of d Metals Key: X = F ® I; X I = F ® Br; X II = F, CI; X III = CI ® I and the same applies to CuX. On the other hand, all Cu II halides are known except the iodide. In this case, Cu + oxidises I – to I : 2Cu 4I Cu I I s However, many copper (I) compounds are unstable in aqueous solution and undergo disproportionation.
2Cu + ® Cu + + Cu The stability of Cu + (aq) rather than Cu + (aq) is due to the much more negative D hyd H o of Cu + (aq) than Cu + , which more than compensates for the second ionisation enthalpy of Cu. The ability of oxygen to stabilise the highest oxidation state