Abstract:
The discovery of transition metal complexes of stoichiometries different from those expected on purely valence grounds led Werner to propound his
classical co-ordination theory, therein recognising in these metal atoms a property
of great significance to chemistry, and which even now is not fully understood.
Through the study of geometrical and optical isomers Werner and later
workers deduced the importance of specifio stereochemical arrangements in these
molecules and their nature has been a subject of enquiry ever since. Not only
has it been repeatedly found that the lighter transition metals exhibit tetrahedral,
planar and octahedral stereochemistry but it has also become clear that
certain metals prefer a specific arrangement and will not normally adopt another -
leading to the set of generalisations which maintain for example that "nickel
is never tetrahedral."
The application of physical techniques and the growth of quantum chemistry
allowed a more rigorous view of these observations and led to the valence bond
theory of Pauling. This theory, particularly when later modified by Taube,
laid great emphasis on covalent bonding between metal and ligand and consequently
on the highly directional nature of the atomic orbitals involved. These were
the hybridised orbitals of the metal which in the particular combinations sp3,
dsp2 and d2sp3 accounted for the generally observed tetrahedral, planar and
octahedral stereochemistry of complexes. Tne theory furthermore provided an explanation
for many of the magnetic properties of these compounds and, in a general
way, for their reactivity so that a strong impression grew that the stereochemistry
of any complex was simply a function of the orbital hybridisation of its
central metal atom.
