Synthesis Hybrid molecular models for coordination compounds

SYNTHESIS HYBRID MOLECULAR MODELS FOR COORDINATION COMPOUNDS 

Another important manifestation of the ligand influence is the thermochemistry of the isomers of the CCs. The problem of isomerism of the CCs was a crucial 

In the previous chapters, we developed an approach which can be used to put the process of developing mechanistic descriptions of PES (i.e. of developing MM force fields) on a rational basis. Deductive molecular mechanics [2-4] (DMM) allows us to develop a form of the MM force fields to analyze the form of the electronic wave function relevant to the physical picture of the electronic structure of the considered class of molecules. In this chapter we apply the previously developedDMM approach to analytical derivation of the QM based form of the force fields involving the nontransition metal atoms. 

The fundamental reasons for the difficulties faced by the MM methods when metal (both transition and nontransition) complexes are involved can be understood if one does not consider the MM as a purely empirical scheme (as it is frequently done), but think about them as of some reflection of specific features of molecular electronic structure, formalized by the form of the trial wave function of that class of compounds where such a parameterization might be possible. As shown in Chapter 3, organic compounds for which the MM methods are known to demonstrate significant successes can be described by the QC method, which directly leads to local and transferable two-center bonds. It is shown in Chapter 3 that the derivation of the MM method from the QC description is possible due to a common background of the MM and SLG description, which consists in the physical presence of two-center, two-electron bonds in organic molecules (in strict terms of Section 1.7 - numbers of electrons in each of the geminals weakly fluctuate). 

To cope with the problems of a mechanistic description of CC, we will first analyze three basic questions the nature of the differences in behavior between central atoms on the one hand and organogenic atoms on the other hand, which results in limitations for the MM techniques when applied to molecules of CC. Getting an idea of the source of these differences tentatively allows us to address further questions developing an adequate MM-like scheme for CCs of nontransition metals and non-metals which will be able to reproduce fine structural features of the mutual ligand influence characteristic for this class of molecules. Next we turn to the most complex problem - developing a hybrid modeling technique which would allow us to cover complexes of transition metals with open 7-shells. 

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