Why are electrons active?
For most cmpds of the main group elements the lone pairs are of
equal importance as bond pairs in determining structure hence the
basis of VSEPR theory. The lone pairs are said to be
stereochemically active. But for some higher-coordination
main-group cmpds the lone pair becomes stereochemically inactive.
For example, the [SeCl6]2-, [SeBr6]2-, [TeCl6]2-, and [TeBr6]2-
ions all have regular octahedral structures in the solid state. But
According to VSEPR these AX6E ions should not have a regular
octahedral structure.
For transition metal cmplxs and organometallic cmpds the d e-s
are completely stereochemically inactive: d AOs point between the
ligands (e.g., oct [M(H2O)6]^n+ ions) or are involved in π bonding.
Cr(CO)6 (octahedral), Fe(CO)5 (trigonal bipyramidal), Ni(CO)4
(tetrahedral), but all have 18 e-s: geometry dictated by ligands.
;)