Ligand Close Packing

For example, it was found that in a series of boron compounds, BF2X and BF3X, the fluorine-fluorine distance remained nearly constant for a wide variety of X groups, even if the steric number changed from 3 to 4, as shown in Table 3.7. 8imilar results were obtained for a variety of other central atoms chlorine-chlorine nonbonded distances were similar in compounds in which the central atom was carbon, oxygen-oxygen nonbonded distances were similar when the central atom was beryllium, and so forth.  

Molecule Coordination Number of B B—F Distance (pm) FBF Angle ( ) F--F Distance (pm) 

Source R. J. Gillespie and P. L. A. Popelier, Chemical Bonding and Molecular Geometry, Oxford University Press, New York, 2001, p. 119 Table 5.3, R. J. Gillespie, Coord. Chem. Rev., 2000, 197, 51. 

As expected, the smaller the F—N—F angle, the longer the N—F bond must be. 

In short, the LCP model complements the VSEPR approach whereas VSEPR predicts that a lone pair will cause a smaller bond angle on the opposite side, LCP predicts that the outer atom outer atom distances should remain essentially unchanged, requiring longer central atom-outer atom distances. An atom that is multiply bonded to a central atom has a similar effect, as shown in the following example. 

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Before discussing the AIM theory, we describe in Chapters 4 and 5 two simple models, the valence shell electron pair (VSEPR) model and the ligand close-packing (LCP) model of molecular geometry. These models are based on a simple qualitative picture of the electron distribution in a molecule, particularly as it influenced by the Pauli principle. 

LIGAND-LIGAND INTERACTIONS AND THE LIGAND CLOSE-PACKING (LCP) MODEL... 

Ligand-Ligand Interactions and the Ligand Close-Packing (LCP) Model... 

Gillespie, R.J. (2000). Improving our understanding of molecular geometry and the VSEPR model through the ligand close-packing model and the analysis of electron density distributions. 

Gillespie, R.J. Robinson, E.A. (1998). Molecular geometry of "ionic" molecules A ligand close packing model. Advances in Molecular Structure and Resonance, 4, 1-41. 

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