Electronic Structure of Complex Ions

We will discuss the crystal field model here. It assumes that the bonding between metal ions and ligands is essentially ionic. More specifically, it considers the effect of approaching Ugands upon the energies of electronic levels in transition metal cations. We will apply this model to octahedral complexes. Before doing so, it maybe helpful to review the electronic structure of uncomplexed trcuisition metal cations, originally covered in Chapter 6. 

Werner spent the next 20 years obtaining experimental evidence to prove his theory. (At the University of Zurich, there remain several 

By all accounts, Werner was a superb lecturer. Sometimes as many as 300 students crowded into a hall with a capacity of 150 to hear him speak. So great was his reputation that students in theology and law came to hear him talk about chemistry. There was, however, a darker side to Werner that few students saw. A young woman badgered by Werner during an oral 

In transition metai cations, 3d is iower in energy than 4s. 

Recall (pages 177-178) that in a simple transition metal cation 

---

This model of the electronic structure of complex ions explains why high-spin and low-spin complexes occur only with ions that have four to seven electrons (d4, d5, d6, d7). With three or fewer electrons, only one distribution is possible the same is true with eight or more electrons. 

We start our description of the electronic structure of complexes of lanthanides by the analysis of the free ion energy structure. The relevant Hamiltonian is written as... 

It is evident that the approach described so far to derive the electronic structure of lanthanide ions, based on perturbation theory, requires a large number of parameters to be determined. While state-of-the-art ab initio calculation procedures, based on complete active space self consistent field (CASSCF) approach, are reaching an extremely high degree of accuracy [34-37], the CF approach remains widely used, especially in spectroscopic studies. However, for low point symmetry, such as those commonly observed in molecular complexes, the number of CF... 

A quantity of some usefulness in the discussion of the electronic structures of complexes is the fraction of electronic charge found on the central ion. Consider, for instance, a complex ML +, containing ligands which are neutral molecules. In an ionic model the... 

Luminescence and Electronic Structure of Metal Ion Complexes with Organic... 

It is well known that alkali and alkaline earth cations are very difficult to complex due to the configuration of the rare gas electronic structure of these ions. Fortunately, some specific ligands are known, such as aprotic dipolar solvents (dimethylformamide, sulfolane, dimethylsulfoxide, N-methyl pyrolidone and so on), aminoxides, phosphinoxides, glymes and polyethylene glycols, crown ethers and cryptates, bidentate amines (tetramethyl ethylene diamine, 1,10 phenanthroline, etc. [14, 53, 61, 68]. 

While the NMR study of paramagnetic species in solution is not confined to non-aqueous solvents, the bulk of the work so far has been carried out in organic solvents for reasons of stability. The results have been confined almost exclusively to transition-metal-complex solutes much less attention has been afforded the solvents except when co-ordinated as ligands. In favourable conditions these studies provide information about NMR spectroscopic theory metal-ligand bonding the electronic structure of ligands, ion association, bulk susceptibilities, various kinetic processes, and molecular structures. The topic has been reviewed recently, and current literature is evaluated in the Specialist Reports of the Chemical Society. ... 

Crystal-field theory can be used to explain many observations in addition to those we have discussed. The theory is based on electrostatic interactions between ions and atoms, which essentially means ionic bonds. Many hnes of evidence show, however, that the bonding in complexes must have some covalent character. Therefore, molecular-orbital theory c s > (Sections 9.7 and 9.8) can also be used to describe the bonding in complexes, although the application of molecular-orbital theory to coordination compounds is beyond the scope of our discussion. Crystal-field theory, although not entirely accurate in all details, provides an adequate and useful first description of the electronic structure of complexes. 

The electronic structure of lanthanide ion complexes is governed by two important factors First, due to the large value of Z, the spin... 

In TT-complexes formed from aromatic compounds and halogens, the halogen is not bound to any single carbon atom but to the 7r-electron structure of the aromatic, though the precise geometry of the complexes is uncertain. The complexes with silver ions also do not have the silver associated with a particular carbon atom of the aromatic ring, as is shown by the structure of the complex from benzene and silver perchlorate. ... 

Complex ions used for electroplating are anions. The cathode tends to repel them, and their transport is entirely by diffusion. Conversely, the field near the cathode assists cation transport. Complex cyanides deserve some elaboration in view of their commercial importance. It is improbable that those used are covalent co-ordination compounds, and the covalent bond breaks too slowly to accommodate the speed of electrode reactions. The electronic structure of the cyanide ion is ... 

Until about 20 years ago, the valence bond model discussed in Chapter 7 was widely used to explain electronic structure and bonding in complex ions. It assumed that lone pairs of electrons were contributed by ligands to form covalent bonds with metal atoms. This model had two major deficiencies. It could not easily explain the magnetic properties of complex ions. 

In the Fe(CN)64- ion shown at the left, Ao is so large that the six 3d electrons of the Fe2+ ion pair up in the lower energy orbitals. In this way, the Fe2+ ion achieves its most stable (lowest energy) electronic structure. The complex is diamagnetic, with no unpaired electrons. 

---