What is the structure of anionic coordination sphere in $[Cr(NH_3)_6][CuCl_5]$ complex?

Trigonal bipyramidal

The correct answer is Option (1) → Trigonal bipyramidal

In the coordination complex $[Cr(NH_3)_6][CuCl_5]$, the formula consists of a cationic coordination sphere and an anionic coordination sphere.

1. Identifying the Anionic Sphere

The complex dissociates as follows:

$[Cr(NH_3)_6][CuCl_5] \rightarrow [Cr(NH_3)_6]^{3+} + [CuCl_5]^{3-}$

The anionic coordination sphere is the $[CuCl_5]^{3-}$ ion.

2. Determining the Structure

For a coordination number of 5 (where the metal is bonded to five ligands), the two common geometries are Trigonal bipyramidal and Square pyramidal.

In the specific case of the $[CuCl_5]^{3-}$ ion within this complex:

• The Copper ($Cu^{2+}$) ion has a $d^9$ electronic configuration.
• The $[CuCl_5]^{3-}$ anion is a classic example used in textbooks to illustrate the Trigonal bipyramidal geometry.
• In this geometry, the five chloride ligands are arranged such that three are in an equatorial plane (forming a triangle) and two are at the axial positions (above and below the plane).