The four coordinate geometric parameter: A new quantification of geometry for four coordinate aluminum and gallium

The chemistry of aluminum and gallium compounds has long been a major area of research. Much of this interest stems from the compound’s use as Lewis acid catalysts. Efforts to create better aluminum and gallium catalysts have often focused on the addition of new ligands to increase the Lewis acidity of the metals. However, a four coordinate Lewis acid catalyst generally must add a fifth substituent during its catalytic cycle, so the degree to which its geometry encourages this addition will also affect its activity. To help determine the degree to which a four coordinate compound is geometrically optimized to take on a fifth substituent, we have created the four coordinate geometric parameter (FGCP). The FGCP of a compound can be calculated by a simple formula utilizing the ligand-metal-ligand angles of the structure. A high FGCP value indicates a compound with an open space for a fifth substituent, while a lower FGCP value indicates a compound with a more tetrahedral geometry. A negative FGCP suggests a geometry approaching square planar, which is admittedly very unlikely for an aluminum or gallium compound. The FGCP values have been calculated for a number of structures reported in the literature, demonstrating that the method is useful for quantifying the geometry of a variety of known group thirteen compounds.