Type of isomerism exhibited by compounds $[Cr(H_2O)_6]Cl_3$, $[Cr(H_2O)_5Cl]Cl_2 \cdot H_2O$, $[Cr(H_2O)_4Cl_2

Question

Type of isomerism exhibited by compounds $[Cr(H_2O)_6]Cl_3$, $[Cr(H_2O)_5Cl]Cl_2 \cdot H_2O$, $[Cr(H_2O)_4Cl_2]Cl \cdot 2H_2O$ and the value of coordination number (CN) of central metal ion in all these compounds, respectively, is:

Accepted Answer

1. **Isomerism:** The given compounds differ in the number of solvent (water) molecules directly bonded to the metal ion as ligands versus those present as free solvent molecules in the crystal lattice. This specific type of isomerism is known as **solvate isomerism** (or hydrate isomerism when water is the solvent). 
2. **Coordination Number (CN):** The coordination number is the total number of ligand donor atoms to which the metal is directly bonded. 
- In $[Cr(H_2O)_6]Cl_3$, Cr is bonded to 6 oxygen atoms of water. CN = 6.
- In $[Cr(H_2O)_5Cl]Cl_2 \cdot H_2O$, Cr is bonded to 5 oxygen atoms of water and 1 chlorine atom. CN = 5 + 1 = 6.
- In $[Cr(H_2O)_4Cl_2]Cl \cdot 2H_2O$, Cr is bonded to 4 oxygen atoms of water and 2 chlorine atoms. CN = 4 + 2 = 6.
Thus, in all these compounds, the coordination number of the central metal ion Cr is 6.

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