New insights into the effects of covalency on the ligand field parameters: a DFT study M. Atanasov a,b , C.A. Daul a, * , C. Rauzy a a D epartement de Chimie, Universit e de Fribourg, Perolles, CH-1700 Fribourg, Switzerland b Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bontchev Str. Bl. 11, 1113 Sofia, Bulgaria Received 15 July 2002 Abstract A new, non-empirical, DFT based ligand field (LF) model is proposed. The calculation involves two steps: (i) an average of configuration (AOC), with equal occupation of the d-orbitals is carried out, (ii) with these Kohn–Sham orbitals kept frozen, the energies of all single determinants (SD) within the whole LF-manyfold is performed. These energies are then used to estimate all the Racah- and LF-parameters needed in a conventional LF-calculation. The results of this first-principle prediction are in very good agreement with the experimental values. Test calculation of tetrahedral Cr ðIVÞ and Ni ðIIÞ complexes are used to validate the new model and to analyze the parameters of the LF. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction Ligand field (LF) theory has been used with success to describe ground and excited electronic states originating from d n transition metals (TM) in their complexes. Both crystal field theory (CFT) [1–4] and developments – the angular overlap model (AOM) [5,6] parameterize the Hamiltonian in terms of one-electron (LF) pa- rameters and two-electrons repulsion integrals within the manifold of d-electrons. The latter ones are treated as atomic-like, thus preserving spher- ical symmetry, while the former take full account for the lowering of symmetry when a spherical TM atom or ion is introduced in a complex. Thus, in cubic symmetry, only one energy parameter 10 Dq – the energy difference between the r and p e g and t 2g for octahedral field (or between the r þ p and p t 2 and e orbitals for tetrahedral field) is introduced in addition to RacahÕs interelectronic repulsion parameters B and C. These three pa- rameters have been determined from a fit to electronic absorption spectra in high resolution. From these data, two general observations could be made: 1. Ligands order in a sequence of increasing values of10Dq,calledspectro-chemicalseries[7],where negatively charged ligands such as I < Br < Cl < F possess smaller 10 Dq values, than neu- tral molecules H 2 O < NH 3 < pyridine, CN  and CO being the strongest ligands due to their ability for back bonding. Chemical Physics Letters 367 (2003) 737–746 www.elsevier.com/locate/cplett * Corresponding author. E-mail address: claude.daul@unifr.ch (C.A. Daul). 0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0009-2614(02)01762-1