Identify the ligand and its denticity

EDTA is ethylenediaminetetraacetate, written as $\mathrm{EDTA}^{4-}$. It is a hexadentate ligand, meaning it can donate 6 pairs of electrons to a metal ion:

• 2 from nitrogen atoms
• 4 from carboxylate oxygen atoms

So, when $\mathrm{Ca}^{2+}$ binds with $\mathrm{EDTA}^{4-}$, the metal ion is surrounded by 6 donor atoms.

Determine the coordination number

Since EDTA binds through 6 donor atoms, the coordination number of $\mathrm{Ca}^{2+}$ in this complex is:

$\text{Coordination number} = 6$

Relate coordination number to geometry

The most common geometry for coordination number 6 is octahedral.

Thus, the coordination environment of $\mathrm{Ca}^{2+}$ in $[\mathrm{Ca}(\mathrm{EDTA})]^{2-}$ is:

$\boxed{\text{octahedral}}$

Check options

• A: trigonal prismatic $\rightarrow$ not the usual geometry here
• B: octahedral $\rightarrow$ correct
• C: square planar $\rightarrow$ usually coordination number 4
• D: tetrahedral $\rightarrow$ usually coordination number 4

Final answer

The correct option is:

$\boxed{\text{B: octahedral}}$