Question

The hexaaquomanganese$\left(\mathrm{II}\right)$ion contains five unpaired electrons while the hexacyano ion contains only one unpaired electron. Explain using Crystal Field Theory.

Answer 1

Crystal field theory:

• The net change in crystal energy caused by the configuration of d orbitals of a transition metal cation within a coordinating group of anions, often known as ligands, is described by crystal field theory.
• The ability of transition metals to form complexes is a key property.
• A complex is made up of a core metal atom or ion encircled by a group of ligands.
• Crystal field theory governs the interaction of these ligands with the core metal atom or ion.

Hexaaquomanganese$\left(\mathrm{II}\right)$ion and hexacyano ion:

• Mn$\left(\mathrm{II}\right)$ has an electronic structure $3{\mathrm{d}}^{5}4{\mathrm{s}}^0$.
• The ligand attached to the metal in the scenario of hexaaquamanganese $\left(\mathrm{II}\right)$ ion is water.
• Water is a poor field ligand and is unable to associate up the electrons of the $3\mathrm{d}$ subshell, and the configuration is ${\mathrm{t}}_2{\mathrm{g}}^3\mathrm{and}{\mathrm{eg}}^2$.
• Thus it contains five unpaired electrons.
• In the instance of hexacyano ion, the ${\mathrm{CN}}^{-}$ is a strong field ligand.
• It causes electron pairing in the d orbitals, resulting in only a single unpaired electron in ${\mathrm{t}}_2{\mathrm{g}}^5{\mathrm{eg}}^2$.