LCP theory


In chemistry, ligand close packing theory, sometimes called the ligand close packing model describes how ligand – ligand repulsions affect the geometry around a central atom. It has been developed by R. J. Gillespie and others from 1997 onwards and is said to sit alongside VSEPR which was originally developed by R. J. Gillespie and R Nyholm. The inter-ligand distances in a wide range of molecules have been determined. The example below shows a series of related molecules:
F-F distance O-F distance C-F bond length C=O bond length
CF4216132
O=CF3−216223139123
O=CF2216222132117

The consistency of the interligand distances in the above molecules is striking and this phenomenon is repeated across a wide range of molecules and forms the basis for LCP theory.

Ligand radius

From a study of known structural data a series of inter-ligand distances has been determined and it has been found that there is a constant inter-ligand radius for a given central atom. The table below shows the inter-ligand radius for some of the period 2 elements:
LigandBerylliumBoronCarbonNitrogen
H1109082
C137125120
N144124119
O133119114
F128113108106
Cl168151144142

The ligand radius should not be confused with the ionic radius.

Treatment of lone pairs

In LCP theory a lone pair is treated as a ligand. Gillespie terms the lone pair a lone pair domain and states that these lone pair domains push the ligands together until they reach the interligand distance predicted by the relevant inter-ligand radii. An example demonstrating this is shown below, where the F-F distance is the same in the AF3 and AF4+ species :
F-F distance A-F bond length F-A-F angle
NF3212136.5102.3
NF4+212130109.5
PF323715797.8
PF4+238145.7109.5

LCP and VSEPR

LCP and VSEPR make very similar predictions as to geometry but LCP theory has the advantage that predictions are more quantitative particularly for the second period elements, Be, B, C, N, O, F. Ligand -ligand repulsions are important when