UNSPECIFIED. (2003) Ligand field and density functional descriptions of the d-states and bonding in transition metal complexes. FARADAY DISCUSSIONS, 124 . pp. 379-391. ISSN 1364-5498

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Abstract

The d-orbital energy sequences for low symmetry transition metal complexes derived from Kohn-Sham density functional theory and ligand field theory are different due to each model's treatment of interelectron repulsion. The implications for providing a unified description of the underlying metal-ligand bonding are analysed and illustrated using conventional and time-dependent DFT. Previous detailed spectroscopic studies have established the d orbital sequence in planar coordination complexes containing p-donor halide ligands as d(x2-y2) >> d(xy) > d(xz), d(yz) > d(z2) while for a sigma-only system like [Pd(NH3)(4)](2+), ligand field approaches like the angular overlap model (AOM) or cellular ligand field (CLF) model predict dxz, dyz and dxy should be degenerate. However, the energies of the Kohn- Sham (KS) 'd' orbitals of [PdCl4](2-) and [Pd(NH3)(4)](2+) place d(xy) below the d(xz)/d(yz) pair. Direct use of the KS orbital eigenvalues in AOM or CLF analyses would imply both ligands are pi cceptors. This result is independent of the choice of functional or whether the calculation is carried out in vacuo or in a polarised continuum representing solvation by water. The origin of the difference between the KS and LFT d orbital sequences derives from their treatments of d-d interelectron repulsion. KS orbitals include interelectron repulsion contributions while LFT d orbitals do not. For a low-spin d(8) complex, DFT gives less d-d interelectron repulsion in the xy plane leading to a lowering of dxy relative to d(xz)/d(yz). This differential effect can be reversed qualitatively by progressively removing electron density from dz2 and placing it in d(x2-y2). When about 0.6 electrons is rearranged, E(d(xy)) > E(d(xz)/d(yz)) for [PdCl4](2-) and the d(pi) orbitals for [Pd(NH3)(4)](2+) are virtually degenerate. The wider rami. cations of these interelectron repulsion effects are discussed for other symmetries. Excited d state energies for [PdCl4](2-) are computed using both time dependent density functional theory (TDDFT) and determinant energies. The latter give the experimental sequence (1)A(2g) < E-1(g) < B-1(1g) while TDDFT gives E-1(g) < (1)A(2g) < B-1(1g). Both give transition energies up to 30% lower than observed. A DFT analysis of the bonding energies in [PdCl4](2-) indicates that the Pd-Cl pi bonding in the molecular plane is about 33% weaker than the out-of-plane pi interaction due to non-zero overlap between ligand orbitals. The normal LFT assumption of linear ligation may not always be valid.

Item Type:	Journal Article
Subjects:	Q Science > QD Chemistry
Journal or Publication Title:	FARADAY DISCUSSIONS
Publisher:	ROYAL SOC CHEMISTRY
ISSN:	1364-5498
Date:	2003
Volume:	124
Number of Pages:	13
Page Range:	pp. 379-391
Identification Number:	10.1039/b211341d
Publication Status:	Published
URI:	http://wrap.warwick.ac.uk/id/eprint/9438

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