Relationship between Cationic Charge, Coordination Number, and Polarizability in Oxidic Materials

The Coordination Number of Silicon in Silicon−Oxygen Compounds: The Special Case of 6-Fold Coordination in Thaumasite

In most silicon-oxygen compounds, the silicon atom is 4-fold coordinated. Exceptionally, silicon is found in 6-fold coordination, as in the mineral thaumasite, which also may be formed in the degradation, and sometimes subsequent weakening, of certain concretes. The connection between coordination number and chemical bonding in oxidic compounds generally is explored using the optical basicity concept to estimate the extent of negative charge provided by oxide(-II) donor atoms in the coordination sphere. A correlation is established between this charge and the heat of formation of silicate (per oxide(-II) atom), which indicates that reduction in charge decreases thermodynamic stability. Calculations for thaumasite show that 4-fold coordination would provide a very small charge, but that 6-fold provides a charge comparable with that for stable 4-fold coordinated silicates such as K2Si2O5.

Ionic−Covalent Character of Metal and Nonmetal Oxides

The acid-base properties of oxidic media are quantified in terms of the optical basicity concept, which serves to correlate many properties with chemical constitution. Optical basicity values, Lambda, have been assigned to 25 oxides such that they relate to Lambda for crystalline CaO being taken as unity. Since Lambda for an oxide is proportional to the degree of negative charge borne by the oxide-(-II) atom or ion, it follows that optical basicity should go hand-in-hand with the ionic/covalent nature of the cation-oxide-(-II) bonding. Unfortunately, this assumption produces many anomalies and trends that do not fit normal inorganic trends. The problem is resolved by adjusting the influence of ionic forms to the bonding by taking into account the heats of formation. In contrast to the (Pauling) electronegativity treatment of oxides, this procedure allows assignment of percentage ionicity to the bonding, and the trends in these in the Periodic Table are as expected for inorganic oxides.