Complexes covalent bonding models

A covalent bonding model is needed to account for the effects of Jt-donor or 7i-acceptor ligands and synergic bonding. A molecular orbital description provides a more universal description of bonding within transtion metal complexes. 

Electronic spectral absorptions associated with electronic transitions between d orbitals provide information about the magnitude of crystal field splitting. Anomalies in the results obtained indicate the need for a more covalent bonding model to describe some complexes. 

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. 

Eaq and Caq are the tendency of acid A and base B to undergo ionic and covalent bonding, respectively. Equation (2) resembles that proposed by Drago et al. (18) to model heats of complex formation of acids and bases in solvents of low dielectric constant. Only Lewis acids of ionic radius greater than 1.0 A obey Eq. (2). For all smaller Lewis acids, a third pair of parameters has to be introduced ... 

The mechanism shown in Scheme 5 postulates the formation of a Fe(II)-semi-quinone intermediate. The attack of 02 on the substrate generates a peroxy radical which is reduced by the Fe(II) center to produce the Fe(III) peroxide complex. The semi-quinone character of the [FeL(DTBC)] complexes is clearly determined by the covalency of the iron(III)-catechol bond which is enhanced by increasing the Lewis acidity of the metal center. Thus, ultimately the non-participating ligand controls the extent of the Fe(II) - semi-quinone formation and the rate of the reaction provided that the rate-determining step is the reaction of 02 with the semiquinone intermediate. In the final stage, the substrate is oxygenated simultaneously with the release of the FemL complex. An alternative model, in which 02 attacks the Fe(II) center instead of the semi-quinone, cannot be excluded either. 

A more complete and much more rigorous description of bonding in complexes would be provided by a quantum mechanical treatment. Such a treatment is especially needed in the case of departures from the ionic model and increasing contribution of covalent bonding (ion pairs, soft donors and acceptors). However only a few studies have been reported. They are mainly concerned with cation hydration and use either semi-empirical 19—21) or non-empirical methods 22—24). A non-empirical treatment of cation NH3 systems has also been performed recently (25). However the present state of the computations is still far from providing a complete description of the system including the medium. The latter may be taken into account by a Bom-type "solvaton (27,26). Heats of hydration may then be calculated (27). A discussion of this aspect of the problem is deferred to a later date, awaiting especially a more complete analysis of non-empirical calculations. In the course of the discussion of... 

Chuvylkin et al. (54) have used this approach to discuss EPR signals arising from weak R02 surface complexes in a number of systems where the g tensor does not fit the pattern expected [Eq. (6) and Fig. 3] from the ionic model. This is not discussed quantitatively, but they conclude that the appearance of covalently bonded oxygen is impossible without a favorable orientation of appropriate electronic orbitals. A similar covalent bonding approach has been considered theoretically for the chemisorption of oxygen on silicon surfaces (55). Examples of weakly bonded oxygen are given in Section IV,E. 

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