Coordination in inorganics

Abstract

Coordination number (CN) is discussed from a geometrical point of view. Two models for the determination of CN 8 to 11 have been developed and tested with good results on 78 polyhedra consisting of alkali/alkaline earth metals as central atoms and halogen atoms/water oxygens as ligands. In these models the polyhedral volume (Vol_poly), the volume of a sphere — least-squares fitted to the observed polyhedron (Vol_sph), and the CN itself are used in the calculations in order to optimize and motivate the selected number of ligands. The first model shows that for a certain CN there exists a linear correlation between Vol_poly and Vol_sph. Since the values of the observed polyhedra lay close to the line obtained for ideal polyhedra, it was assumed that the CN is fairly well described. In the second model the Vol_poly/Vol_sph ratio is plotted against the coordination number for observed and ideal polyhedra. With increasing numbers of ligands the observed values exhibit a trend towards a closer adaption to the curve obtained from the ideal polyhedra. This adaption, or optimum CN, can be used as a criterion that motivates the choice of a certain coordination number. Finally, comparisons are made between the second model and the calculated bond distances in some observed polyhedra.