Question 22.3: How many donor groups does this polydentate ligand, nitrilot...
How many donor groups does this polydentate ligand, nitrilotriacetic acid (NTA), have?
Collect, Organize, and Analyze We need to examine this molecular structure to find electron pairs that can be donated. Because there are three single bonds around the N atom, the atom’s fourth sp^{3} orbital must contain a lone pair of electrons. When all three carboxylic acid groups are ionized, there are three carboxylate groups in the molecule, and each carboxylate group can donate one nonbonding pair of electrons from one of its oxygen atoms to a metal atom.
Solve The central N atom and an O atom from each of the three carboxylate groups form a total of four coordinate bonds. Therefore NTA is potentially a tetradentate ligand with four donor groups.
Think About It The tetradentate capacity of NTA is reasonable because, like EDTA, it is an aminocarboxylic acid. It has one fewer amino group and one fewer carboxylic acid group than the hexadentate EDTA.