Low-spin ground state

Even when a distortion cannot be directly detected, it can have deep consequences for the electronic properties of these materials. For example, there is a clear-cut competition between JT effect and the molecular Coulomb correlations. Hund s rules would tend to form a high-spin state that minimizes repulsion between electrons, while JT-split electronic levels imply a low-spin ground state. 

Fig. 13. Energy diagram showing intereonversion of square pyramid and trigonal bipyramid for (a) d6 low-spin ground state and (b) d6 intermediate-spin excited state.

For low-spin octahedral d, the ground state is T2g, that is, (t2g). One hole in t2g is analogous to octahedral d or tetrahedral d . Thus Figure 8 is apphcable, with one spin-allowed d-d transition, "T2g-"Eg, whose energy is equal to Aq. There are quartet excited states, plus the unique sextet state Aig. The vast majority of octahedral Mn° compounds are high-spin iron(ni) complexes have larger Aq values and low-spin ground states are more common, for example, in [Fe(CN)6] and [Fe(bipy)3] +. 

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