Four-coordinate molecules

Figure 8.9 Examples of four-coordinated molecules of beryllium (a) BeCl2 2Et20 and (b) the polymeric molecule BeCl2.

All of the four-coordinated molecules we have discussed so far have tetrahedral geometry around the central atom. Methane, CH4, is the most well known example. It may come as something as a surprise, then, to discover that the tetrachlorplatinum (II) ion [PtCl4]2- has an essentially two-dimensional square-planar configuration. This... 

Figure 4 Distribution of the tetrahedricity measure 6 in liquid water along the liquid-vapour coexistence curve (from 100 to 450 K). Thick lines indicate two coexisting liquid phases. Left panel - total distributions right panel -distributions for four-coordinated molecules.

Figure 5 Temperature dependence of the concentration C of the tetrahedrally ordered four-coordinated water molecules (upper panel) and of the liquid water density (lower panel) along the liquid-vapour coexistence curve. Vertical dashed line indicates the temperature of the liquid-liquid transition. Dotted lines indicate the densities and concentrations of the coexisting phases. Stars indicate percolation transition of the tetrahedrally ordered four-coordinated molecules.

Lead appears to be able to interact with complex small biomolecules as well, such as flavins for example, bis(lO-methylisoalloxazine) perchlorate tetrahy-drate (223). IsoaUoxazine is a planar three-ringed heterocychc amino cofactor associated with riboflavin and is active in oxidation-reduction reactions with metals such as Mo and Fe. Lead binds to bis(lO-methylisoalloxazine) in a 1 1 metal-ligand complex, with two additional waters bound resulting in a four coordinate molecule with a total of four oxygen donors. An active lone pair results in a distorted square-pyramidal structure. As is the case for citrate, extensive hydrogen bonding was observed in the crystal lattice. 

This has been a fruitful area of study. Both kinetic and structural data have continued to illuminate the reaction pathways of stable five-coordinate species themselves, and the nature of five-coordinate intermediates or transition states encountered from associative reactions of four-coordinate molecules. 

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