Complex ions crystal field model

Crystal field theory is a simple electrostatic model that treats the transition metal ion and the ligands as point charges. It is very successful in predicting in a very simple way the role of the 3d electrons in determining the properties of complex ions. Crystal field theory ignores the fact that the ligands form coordinate bonds with the transition metal ion via their lone pairs. 

The general influence of covalency can be qualitatively explained in a very basic MO scheme. For example, we may consider the p-oxo Fe(III) dimers that are encountered in inorganic complexes and nonheme iron proteins, such as ribonucleotide reductase. In spite of a half-filled crystal-field model), the ferric high-spin ions show quadrupole splittings as large as 2.45 mm s < 0, 5 = 0.53 mm s 4.2-77 K) [61, 62]. This is explained... 

Vanadium(n) Complexes.—Dehydration of VSO. THjO has been shown to proceed via the formation of VS04,mH20 (where n = 6, 4, or 1) and V(OH)-(SO4), which were characterized by X-ray studies. The polarographic behaviour and the oxidation potential of the V -l,2-cyclohexanediamine-tetra-acetic acid complex, at pH 6—12, have been determined.Formation constants and electronic spectra have been reported for the [Vlphen),] " and [V20(phen)] complexes. The absorption spectrum of V ions doped in cadmium telluride has been presented and interpreted on a crystal-field model. The unpaired spin density in fluorine 2pit-orbitals of [VF ] , arising from covalent transfer and overlap with vanadium orbitals, has been determined by ENDOR spectroscopy and interpreted using a covalent model. " ... 

The NiCIi ion, a representative tetrahedral complex, has been studied in great detail spectroscopically at 2.2 K. The observations can all be accounted for by a parameterized crystal field model, with A, 3500 cm-1. 

I Ma) in a metal complex is borrowed, according to the crystal field model, from an electric-dipole aUowed transition or transitions of the metal ion, Mo) -> IM ) or IMg) IMk), at higgler energy. 

Thus far we have considered the crystal-field model only for complexes having an octahedral geometry. When fliere are only four ligands about the metal, the geometry is generally tetrahedral, except for the special case of metal ions with a d electron configuration, which we will discuss in a moment The crystal-field splitting of the metal d orbitals in tetrahedral complexes differs from that in octa-... 

We have seen that the crystal-field model provides a basis for explaining many features of transition-metal complexes. In fact, it can be used to explain many observations in addition to those we have discussed. Many lines of evidence show, however, that the bonding between transition-metal ions and ligands must have some covalent character. Molecular-orbital theory (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. The crystal-field model, although not entirely accurate in all details, provides an adequate and useful first description of the electronic structure of complexes. 

This is why our discussion of crystal and ligand theories has been exemplified by complexes formed by elements of the first transition series. Had we included spin-orbit coupling within the crystal field model (and in a more complete treatment this would have been done), within the crystal field model it would have appeared with its free-ion value. Not surprisingly, in ligand field theory it becomes a parameter which, characteristically, is found to have a value somewhat lower than that found for the free ion. 

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