The Metal-Ligand Interactions

In the absence of general quantitative methods, we need to resort to qualitative descriptions and models to describe the changes which result from co-ordination of a ligand to a metal. Let us now consider the detailed manner in which a metal may alter the properties of a co-ordinated ligand. 

Metals and Ligand Reactivity, New Edition. Edwin C. Constable Copyright 1996 VCH Verlagsgesellschaft mbH, Weinheim ISBN 3-527-29278-0 

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MOMEC is a force field for describing transition metal coordination compounds. It was originally parameterized to use four valence terms, but not an electrostatic term. The metal-ligand interactions consist of a bond-stretch term only. The coordination sphere is maintained by nonbond interactions between ligands. MOMEC generally works reasonably well for octahedrally coordinated compounds. 

Fabbrizzi L., Licchelli M., Taglietti A., The Design of Fluorescent Sensors for Anions Taking Profit from the Metal-Ligand Interaction and Exploiting Two Distinct Paradigms, J. Chem. Soc., Dalton Trans. 2003 3471-3479. 

However, for the p2, four-electron Ir(HCCH)+ complex, the metal-ligand interactions are so strong that a better NBO description corresponds to either of the two following equivalent metallacyclopropene insertion products 66... 

The metal-ligand interaction in S complexes should he comparable to that in other complexes with sulfur-containing ligands, at least for higher values of n. Additionally, the observation that polysulfides with even n prefer coordination to closed-shell metal ions and those with odd n to open-shell ones indicates that the kind of metal-to-ligand interaction is obviously not restricted to the sulfur atoms attached directly to the metal. 

Experimental Conditions that Affect the Metal-ligand Interactions. . 153... 

Some of the above mentioned problems can be reduced by applying pretreated samples to the column. The samples may be partially digested or passed through a precolumn It is therefore essential to provide information showing that the metal-ligand interaction is not disrupted during the pre-separation steps and no change in the state of the metal has occured. 

The s and f block elements present a particular challenge in the molecular mechanics field because the metal-ligand interactions in both cases are principally electrostatic. Thus, the most appropriate way to model the M-L bonds is with a combination of electrostatic and van der Waals nonbonded interactions. Indeed, most reported studies of modeling alkali metal, alkaline earth metal and rare earth complexes have used such an approach. 

Since the interpretation of NMR parameters in complex biological systems is considerably facilitated by the use of model compounds it is evident that a proper understanding of the metal-ligand interactions in the synthetic counterparts of vitamin B12, which are the subject of this review, can yield additional details and help clarify the role of corrinoids in Nature. 

Additional uncertainty arises from the inadequacy of the model itself, both specific and nonspecific. The nonspecific refers to the failure of the angular overlap model itself to adequately represent the electronic states of the molecule, and in particular, the inconsistencies in AOM parameter values that result. This can only be assessed by compiling enough data to evaluate those inconsistencies. Specific defects are those in which the metal-ligand interaction is treated unsatisfactorily, but can be improved within the framework of the model. An example is when the 7t-interaction is treated as isotropic, but is actually anisotropic. Specific defects can be assessed immediately by making the proposed alteration, and this works particularly well if no additional parameters are required to make the alteration. 

The sequence of energy levels obtained from a simple molecular orbital analysis of an octahedral complex is presented in Fig. 1-12. The central portion of this diagram, with the t2g and e levels, closely resembles that derived from the crystal field model, although some differences are now apparent. The t2g level is now seen to be non-bonding, whilst the antibonding nature of the e levels (with respect to the metal-ligand interaction) is stressed. If the calculations can be performed to a sufficiently high level that the numerical results can be believed, they provide a complete description of the molecule. Such a description does not possess the benefit of the simplicity of the valence bond model. 

Before leaving this brief introduction to molecular orbital theory, it is worth stressing one point. This model constructs a series of new molecular orbitals by the combination of metal and ligand orbitals, and it is fundamental to the scheme that the ligand energy levels and bonding are, and must be, altered upon co-ordination. Whilst the crystal field model probably over-emphasises the ionic contribution to the metal-ligand interaction, the molecular orbital models probably over-emphasise the covalent nature. 

In this chapter we have briefly discussed the models which have been developed for the description of co-ordination compounds, and have stressed the consequences that the metal-ligand interaction may have for the ligand. In the remainder of this book we will consider the detailed chemical consequences of the metal-ligand interaction. The suggestions for further reading given below will allow the interested reader to delve deeper into co-ordination chemistry should he or she so wish. 

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