The 10 Dq parameter

In an octahedral coordination site, the electrostatic field produced by the six ligands (represented as point negative charges and interacting with an electron in the vicinity of the central cation) is expressed by the potential 

The potential, interacting with 3d electrons in the t2 and eg orbitals leads 

rS is the mean value of the fourth power of the radial distance of a 3d orbital from the nucleus. Although accurate estimates of rS are not yet available, its value is assumed to be approximately constant for cations of similar valence in the same transition series. Combining eqs (2.12) and (2.15), the product of D and q is (Zte2/6/ 5) r4 , so that 

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