The coordination environment of Ca²⁺ ion in its complex with EDTA⁴⁻ is:

🧠 Ca²⁺ + EDTA = 6-Coordinate

EDTA (ethylenediaminetetraacetate) is hexadentate: 2 amine N + 4 carboxylate O. When EDTA wraps a single metal ion, the metal is necessarily 6-coordinate octahedral.

Ca²⁺ is large enough to accept all 6 donors → octahedral $\mathrm{[Ca(EDTA)]^{2-}}$.

🗺️ Eliminate the Wrong Geometries

• (1) Tetrahedral — only 4 sites, but EDTA needs 6. ✗
• (2) Trigonal prismatic — 6-coordinate but only seen for very specific d⁰/d¹⁰ ions (W, Mo); not Ca²⁺. ✗
• (3) Octahedral — 6 sites, fits EDTA perfectly. ✓
• (4) Square-planar — only 4 sites. ✗

⚡ The "EDTA → Octahedral" Default

For any small-to-medium-sized metal, EDTA gives an octahedral chelate. Only for very large metals (lanthanides) does the geometry expand beyond 6.

⚠️ Don't Confuse Ca with Pt or Pd

Square-planar geometries belong to d⁸ ions (Pt²⁺, Pd²⁺, Ni²⁺ + strong field). Ca²⁺ is $\mathrm{d^0}$ (a main-group ion in the s-block) — never square-planar.

$\boxed{\text{Answer: (3) octahedral}}$