geometry of transition metals
Transition metal ions, however, present a quite different case.
The transition elements, i.e. those between Sc and Zn and their congeners
"below", are so named because in each of the three series their outer d oribtals
undergo a "transition" in type as nuclear charge increases. The d-orbitals are
initially of high energy (and unoccupied in the neutral atoms of groups IA-IIIA)
but progressively stabilise across each of the series, becoming filled and essentially
part of the core in groups IIB, etc.; in between these two extremes, the d-orbitals
are the principal valence orbitals. The d subshell is only partially filled in
the transition elements themselves, which property can profoundly influence the
geometry of their complexes. The observed geometries are not explicable
in terms of VSEPR theory.