ethylene diamine tetraacetic acid (EDTA)
Each of the acetic acid groups can give up a proton, yielding an anion with a -4 charge. In addition, each of the two nitrogens has a pair of non-bonding electrons. So, in this most basic form, the EDTA anion has six points at which it can attach to a central atom. It does so by wrapping itself around the central atom as shown in the next figure. Note that the central atom is octahedrally coordinated.
ethylenediaminetetraacetatocalcium ion
or just
[Ca(EDTA)]2-
To help you visualize what is going on, here is an interactive model of the same complex ion. The EDTA molecule is rendered as heavy sticks. For clarity, and to emphasize what the backbone of the EDTA molecule is doing, hydrogens are initially hidden from view. You can turn display of the hydrogen atoms on and off using the buttons to the right of the model (may not work properly under Windows95).
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Show hydrogen atoms:
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