One ligating molecule or ion may donate more than one of the six
electron pairs. Each carbonate ion provides two electron pairs in
the complex ,
and each molecule of ethylenediamine, ,
does the same in
("en" is the standard abbreviation for ethylenediamine). These and
other bidentate ("two-toothed") and tridentate ligands are shown
at the right. In the
complex shown, each ethylenediamine molecule bends back to occupy
two of the six octahedral positions.
There are two mirror-related ways of doing this, which leads to
left-handed and right-handed complexes. Only two molecules of diethylenetriamine
are needed to enclose a transition-metal ion completely, and one
ethylenedi-aminetetraacetate ion (EDTA) provides all six bonding
electron pairs. EDTA completely encloses a metal ion in a molecular
cocoon, and its attraction for metal ions is so strong that it will
tear them away from many enzyme molecules. EDTA is used as a means
of removing the last traces of metals from solutions in analytical
and biochemical experiments where their presence would be confusing
or harmful.