The existence of cations of high charge, >3+, is energetically unfavorable and a

Question

The existence of cations of high charge, >3+, is energetically unfavorable and as a result, seldom found in simple ionic materials.    In fact, the vast majority of simple ionic salts contain cations with charges of <2+.   Yet, high oxidation states are readily found in coordination compounds and solid state metal oxides, which can be viewed as solid state coordination complexes since the metal ions are surrounded by oxide ions.    For example, the coordination complex [Fe(H2O)6]Cl3 and the iron oxide, Fe2O3 (a form of rust called hematite) both contain Fe3+ .   There are similar examples with other metals.    Explain why it is more likely to find metals in high oxidation state in metal complexes.

Explanation / Answer

Iron has atomic no. 26. It has 26 electrons. When iron is not co ordinated to ligand all the five d orbitals have same enrgy.(degeneeate)

When it forms a complex the d orbital are splitted and and forms t2g and eg orbital. Now consider Fe2+ system. it will have 24 electrons . the 4 will be in d orbitals.

They will be distributed as t2g-3 and eg-1

Lests calculate cfse for this

Cfse= -0.4* 3 + 0.6*1

= -0.6

Similarly for Fe3+ system

Cfse= -0.4*3 + 0.6 *0

= -1.2

It is clear that cfse is higher (less negative)

For Fe3+ in complex. So due to higher cfse complex bof Fe3+ become more stable and are generally favoured.

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