Silicon, five-fold coordination

As previously pointed out by Lasaga and Gibbs (1990), there is reason to believe that the hydroxide exchange reaction between water and quartz proceeds by way of a fivefold coordinate silicon intermediate. The existence and nature of this five-fold coordinate silicon atom was further investigated by Kubicki et al. (1993). They determined the gas-phase reaction path of the addition of hydroxide to orthosilicic acid and a subsequent ab stracti on of H2O. 

The computation was performed up to the MP2/6-31G(d) level. They gathered evidence suggesting that the five-fold coordinate silicon structure may be a long-lived intermediate in basic solutions and can possibly be observed experimentally (Kinrade et al. 1999). The technique they used in finding the transition state was primarily constrained optimizations followed by Bemy optimization. 

Figure 12.4. Example of a continuous random network of tetrahedrally coordinated atoms. This particular structure is a model for amorphous silicon, it contains 216 atoms and has only six mis-coordinated atoms (five-fold bonded), which represent fewer than 3% defects.

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