10. Playing with a Full Deck:
       The Periodic Table
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       d Orbitals in Bonding

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.

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