The microscopic origin of the strong dependence of the crystal field splitting parameter, 10Dq, upon the metal-ligand distance, R, has been explored using vibronic theory as well as analysing the forces on ligands due to electronic density changes. Both different but complementary methods support that for an octahedral transition metal complex the R dependence of 10Dq mainly comes from a fine detail in the e(g) (similar to x(2) - y(2), 3z(2) - r(2)) wavefunction: the small admixture of s valence ligand orbitals with the corresponding p orbitals. The implication of this result as regards the Stokes shift involved in the luminescent emission is also underlined. (c) 2006 Elsevier B.V. All rights reserved.