Crystal field splitting in octahedral coordination

T able 2.1. Electronic configurations of the first-series transition elements 

In the crystal field model, the split 3d orbital energy levels are assumed to obey a centre of gravity rule. As a result, the three t2g orbitals are lowered by 0.4Ao below, and the two eg orbitals raised by 0.6Ao above, the baricentre. This follows from a simple algebraic argument that the energy of six electrons in the three t2g orbitals is compensated by the energy of four electrons in the two eg 

Consider the elements of the first transition series in octahedral coordination. Ions with one, two or three 3d electrons (for example, Ti3+, V3+ and Cr3+, respectively) each can have only one electronic configuration, and the electrons occupy 

It can be seen from table 2.2 that transition metal ions with 3d5, 3cP and low-spin 3 configurations acquire significantly higher CFSE s in octahedral coordination than other cations. Therefore, ions such as Cr3+, Mn4+, Ni2+, and Co3+ are expected to show strong preferences for octahedral coordination sites. Cations with 3d°, 3d10 and high-spin 3d5 configurations, such as Ca2+, Zn2+, Mn2+ and Fe3+, receive zero CFSE in octahedral coordination. 

If there is competition between pairing energy and crystal field stabilization energy, a cation may be unstable towards oxidation and reduction. For example, the Mn3+ (3d ) ion is readily oxidized to the Mn(IV) oxidation state and is easily reduced to the Mn2+ ion. In addition, the Mn3+ ion disproportionates in aqueous solution  

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