Q. Illustrate crystal field splitting in octahedral complexes?
Initially in a free ion state (I) all the orbitals are in degenerate state. In state (11), all the five orbitals are raised m energy when surrounded uniformly by negative charges. State (111) shows how the degeneracy is removed in an octahedral field. The set of lower three orbitals equal in energy in an octahedral field is given symbols t2g while the upper two equal-energy levels are denoted by e,. The difference in energy between t2g and eg sets is given by a symbol D0, (delta octahedral). Comparing states (11) and (III), we find that the only difference between them is that instead of uniformly distributing the six negative charges (as shown in state II), we have put the six charges at the comers of a regular octahedron. Just by the redistribution of charges the total energy of the system should not change. However, at the same time we say that in state (111). the t2g set of orbitals will be lower in energy and the eg set will be higher. The two facts can be reconciled if the total lowering of 12 set is equal to the total raise in energy of eg orbitals which would give no resultant change in energy as compared to state (11). Of the five orbitals, three are lowered in energy and two are raised in energy. Thus each orbitals is lowered in energy by 2/5D0, causing a total lowering of 615 & the other two orbitals are each raised in energy by 3/5D0, causing a total raise of 6/5 D0,. The sum total of lowering and raise, being equal and opposite, turns out to be zero or no change. The D0, values are generally in the range of 100-250 kJ mol-1.