Transition Elements: Question 10

Worked solution

Step 1: Count the coordinate bonds and coordination number

$\text{EDTA}^{4-}$ has six donor atoms in total: two nitrogen atoms (each with a lone pair) and four oxygen atoms (one from each of its four carboxylate, $-\text{COO}^-$, groups). Each donor atom forms one coordinate (dative covalent) bond to the central metal ion, so a single $\text{EDTA}^{4-}$ ion forms 6 coordinate bonds with $\text{Pb}^{2+}$, giving lead a coordination number of 6. Because $\text{EDTA}^{4-}$ alone supplies all six bonds, only one $\text{EDTA}^{4-}$ ion is needed per $\text{Pb}^{2+}$ ion (a 1:1 complex).

Step 2: Determine the overall charge

The overall charge of the complex is the sum of the metal ion’s charge and the ligand’s charge: $(\text{charge on Pb}^{2+}) + (\text{charge on EDTA}^{4-}) = (+2) + (-4) = -2$

So the complex ion formed, $[\text{Pb}(\text{EDTA})]^{2-}$, carries an overall charge of $2-$.

Step 3: Evaluate each option

• A: correctly gives 6 coordinate bonds, coordination number 6, and overall charge $2-$. This matches the reasoning above.
• B: incorrect, it omits the two nitrogen donor atoms, undercounting both the coordinate bonds and the coordination number as 4 instead of 6.
• C: correctly counts the coordinate bonds and coordination number, but the charge is wrong, it uses only the ligand’s charge ($4-$) without combining it with the $+2$ charge of $\text{Pb}^{2+}$.
• D: incorrect, this treats $\text{EDTA}^{4-}$ as bidentate (like 1,2-diaminoethane) rather than hexadentate, giving far too low a coordinate-bond count and coordination number.

Final answer

• A single $\text{EDTA}^{4-}$ ion forms 6 coordinate bonds with $\text{Pb}^{2+}$, giving coordination number 6 and an overall complex charge of $\mathbf{2-}$, i.e. $[\text{Pb}(\text{EDTA})]^{2-}$, option A.