Chemistry Reference
In-Depth Information
with four to six most common; for f-block metals coordination numbers above six
and up to fourteen may be observed, with seven to nine most common.
Shapes predicted by a simple amended VSEPR model are observed, but some others are
found as well. Inter-conversion between shapes in a particular coordination number
can occur, and many complexes are non-ideal in shape, showing distortion away
from one limiting shape towards another.
Isomerism - the presence in a particular complex of a number of different spatial
arrangements of atoms - is common in complexes.
The two key types of stereoisomers are positional or geometric isomers (such as
cis
and
trans
) and, where a complex is asymmetric, optical isomers (such as
and
). The general class of isomers is diastereomers; where a complex is asymmetric
or dissymmetric (optically active), a diastereomer will have a non-superimposable
mirror image, called an enantiomer.
The absence of any plane of symmetry is a key requirement for a diastereomer having
an enantiomer (and hence being chiral). Thus 'flat' molecules, such as those of
square-planar shape, do not exist as optical isomers; octahedral complexes may be
chiral, depending on the type and arrangement of ligands.
The possible existence of a set of geometric isomers does not mean that all are seen
experimentally. They will differ in strain energy and thermodynamic stability, and
as few as a single one may be isolable.
Ligand shape has an impact on the resultant complex shape, the number of possible
isomers that could form, and the thermodynamic stability of complexes formed.
Apart from formation of coordination complexes, some molecules bind other molecules
or complexes by weaker noncovalent means, such as strong hydrogen-bonding,
forming outer-sphere (host-guest) complexes.
Further Reading
Atkins, P., Overton, T., Rourke, J.
et al.
(2006)
Shriver and Atkins Inorganic Chemistry
, 4th edn,
Oxford University Press. This popular but lengthy general text for advanced students contains
some clearly-presented sections on shape and stereochemistry of coordination complexes.
Clare, B.W. and Kepert, D.L. (1994) Coordination numbers and geometries, in
Encyclopaedia of
Inorganic Chemistry
, vol. 2 (ed. R.B. King), John Wiley & Sons, Ltd, Chichester, UK, p. 795. A
detailed and readable review, replete with examples.
Fergusson, J.E. (1974)
Stereochemistry and Bonding in Inorganic Chemistry
, Prentice-Hall, New
Jersey, USA. An old but valuable resource topic for advanced students, but the depth and detail
may concern others.
Steed, J.W. and Atwood, J.L. (2009)
Supramolecular Chemistry
, 2nd edn, John Wiley & Sons, Ltd,
Chichester, UK. This up-to-date but more advanced and lengthy text provides a comprehensive
coverage of the field, for those who wish to stray beyond the introductory level.
Vogtle, F. (1993)
Supramolecular Chemistry: An Introduction
, John Wiley & Sons, Ltd, Chichester,
UK. An ageing but still very readable account for the student of supramolecular compounds, as
distinct from coordination compounds, that will assist student understanding of this field.
von Zelewsky, A. (1996)
Stereochemistry of Coordination Compounds
, John Wiley & Sons, Inc.,
New York, USA. A mature but detailed coverage of topology in coordination chemistry, with a
good many clear illustrations; a useful, though more advanced, resource topic.
