Selectivity and sensitivity of metal determination by co-ordination compounds.

Quantum chemical structure calculations for metal co-ordination compounds with various organic ligands allow choice of generalized parameters of chelate electronic structures, to form a basis for systematization and prediction of analytical properties. The extent of the co-ordinative saturation of a metal is measured as the sum of the covalent bonding energies of the two-centre metal interactions with the ligand atoms. The concept of "valence state of the ligand" is considered and characterized by the energy sum of the covalent components of two-centre interactions in the ligand. It is shown that the ligand structure can be correlated with the complex stability and this provides a new mechanism for assessing the influence of substituents in ligands. The calculated data make it possible to predict compound stability in solutions and synergic action, and therefore such analytical properties as sensitivity and selectivity. For the azo dyes as an example, it is shown that quantum chemical calculations can explain many of the experimental data on the use of azo dyes in photometry, and suggest directions of search for new analytical reagents.