Untitled - Kelly Walsh High School

Chemistry of the Elements 287
Don’t Forget!
A complex ion will have a central atom, normally a transition metal, with one
or more ligands. A ligand is a Lewis base that reacts with the central atom. The
most common numbers of ligands are four and six. If a species does not contain
a lone pair of electrons, it cannot be a Lewis base, which means it cannot be a
ligand. Some ligands are chelating ligand. A chelating ligand behaves as a Lewis
base more than once to the same central atom. An example of a chelating ligand
is ethylenediamine, NH 2CH 2CH 2NH 2. This molecule donates a pair of electrons
from each of the nitrogen atoms. Three ethylenediamine ligands count the
same as six “normal” ligands. Another common chelating ligand that behaves
like ethylenediamine is the oxalate ion, C 2O 4 2 . The ligand ethylenediaminetetraacetate,
EDTA 4 , chelates by donating six electron pairs. The six electron
pairs make this ligand equivalent to six “normal” ligands.
Crystal Field theory allows predictions concerning the behavior of complexes.
When six ligands are present, the complex formed is usually octahedral. If there
are four ligands present, the complex formed may be either tetrahedral or
square planar. Each of these three geometries has a characteristic splitting of
the d-orbitals predicted by Crystal Field theory. Your textbook will have diagrams
of these characteristic patterns. The steps in utilizing these patterns begin
with determining the number of ligands. If the number is four, there must be
additional information available in order for you to determine if the complex is
tetrahedral or square planar. Once you have the appropriate pattern you should
enter the d-electrons from the metal into the pattern. If there is an option on
how to do this, there must be additional information in the problem.
When a transition metal forms a cation; it is the s-electrons that leave first. Thus,
iron is [Ar]4s 2 3d 6 , and Fe 2 is [Ar]3d 6 , and Fe 3 is [Ar]3d 5 .
19-9 Utterly Confused About Complex Ions
The general equation for the formation of a complex is:
M x L K [M L x]
The equilibrium constant expression, K f, is
K f [M L x ]
[M] [L] x