The Role of d Orbitals

An important question concerns the kind of relationships that exist between the dissociation energy, Dq, and various other molecular properties. What are the main factors responsible for the chemical bonding How does Dq depend upon the charge on the heavy atom Does the HOMO-LUMO interaction of the fragments determine the magnitude of Dq And what is the role of d orbitals on higher elements ... 

Structures involving tetracovalent sulfur (279) have been used to represent this class of compound but the role of d-orbitals in their bonding is not clear (see Section IV,A). 

Ligand field, like crystal field, theory is confined to the role of d orbitals, but unlike the crystal field model, the ligand field approach is not a purely electrostatic model. It is a freely parameterized model, and uses and Racah parameters (to which we return later) which are obtained from electronic spectroscopic (i.e. experimental) data. Most importantly, although (as we showed in the last section) it is possible to approach the bonding in d-block metal complexes by using molecular orbital theory, it is incorrect to state that ligand field theory is simply the application of MO theory. ... 

The Role of d Orbitals. Fig. 4.7 shows a schematic energy-level diagram... 

The differences between the apicophilicity scales summarized above results from the overemphasis of d-orbital contributions in the ab initio study. Thus strong (p- d) Jt-donor ligands, NHj and BHj, have artificially high equatophilicity. Nevertheless, important conclusions may be drawn as to the role of d-orbitals. 

It is important to distinguish between the role of d-orbitals as hybridization functions and their role as polarization functions. For the former case, the d-orbital serves to increase the number of distinct orbitals utilized in the wave-function . Thus, the octet and expanded valence-shell structures (1) and (2) have Qs p, F3 -HCjSp, Fj and CjS pM, F, +C2sp d, Ft valence-state configurations for the trivalent and quadrivalent sulphur atoms. For stmctures (4) and (5) the sulphur valence-state configurations are sp, Ft and sp d, . When a 3d-orbital... 

Apart from two exceptions (refs. 30, 34), calculations of electronic transition energies of thiophen have involved PPP tt approximations (refs. 11, 13, 16, 95), but results from the latter studies do not differ much from those from the former. (For the role of d,-orbitals in spin-orbit coupling in thiophen see ref. 96.)... 

---