Question

what is special abotut the 5- and 6- coordinate geometries?

Answer 1

Because of the number of valence electrons. 5- coordinate geometries have 10 valence electrons while 6- coordinate geometries have 12. For example PCl₅ there are 5 P-Cl σ bonds for a total of 10 valence electrons for the central P atom; in SF₆ there are 6 S-F σ bonds for a total of 12 electrons in the valence shell of S. This is true for any five or six coordinate compounds of a main group element and as such they don't obey the octet rule.

Their geometries are also closely comparable, if we remove a ligand from 6-coordinate geometry we get 5-coordinate geometry as shown below, this is for understanding purpose only, not in reality.

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