Hole size is a fundamental parameter of macrocyclic ligands which may
influence very greatly the properties of resulting metal complexes relative to
those of the corresponding open-chain analogues. While there is now extensive
documentation of hole-size variation affecting properties, by comparison, there
has been much less attention directed toward closely defining the factors which
influence hole size in macrocyclic systems. This situation is so even though a
very considerable amount of X-ray structural data for macrocyclic ligands and
their metal complexes have now been published and may be conveniently re￾trieved from the X-ray crystallographic data bases.