Geometries, preferred, VSEPR

The molecular geometry of a complex depends on the coordination number, which is the number of ligand atoms bonded to the metal. The most common coordination number is 6, and almost all metal complexes with coordination number 6 adopt octahedral geometry. This preferred geometry can be traced to the valence shell electron pair repulsion (VSEPR) model Introduced In Chapter 9. The ligands space themselves around the metal as far apart as possible, to minimize electron-electron repulsion. 

The preferred geometries of carbenium ions and carbanions are correctly predicted by the valence shell electron pair repulsion (VSEPR) theory. The theory is general and can be applied to organic and inorganic compounds, regardless of charge. 

The most detailed structural work on pentacoordinate structures has been carried out and compiled by Holmes, and some of his findings are summarized here. Basic VSEPR rules can account qualitatively for geometries with simple acyclic ligands for example, the series Me PF5 (n = 1 -3). However, as ligands become more complex, other factors predominate. Preference rules have been derived for both the tbp and sp geometries and are listed here in order of probable importance. 

Comment (a) The 1 ion is one of the few structures for which the bond angle (180°) can be predicted accurately even though the central atom contains lone pairs, (b) In C2H4, all six atoms lie in the same plane. The overall planar geometry is not predicted by the VSEPR model, but we will see why the molecule prefers to be planar later. In reality, the angles are close, but not equal, to 120° because the bonds are not all equivalent. 

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