Answer :
The metal ion to which the ligands exits attached at the centre of a
coordination complex.
What is a central metal ions?
The core atom or ion in a coordination entity is the one to which a predetermined number of other ions or groups are coupled in a specific geometrical arrangement surrounding it.
Central metal ions in the complex compounds have vacant d orbitals. They accept the lone pair of electrons from ligands. Therefore, they act as Lewis acids.
When a solution contains [tex]$\mathrm{Hg}^{2+}$[/tex] and [tex]$\mathrm{I}^{-}$[/tex], depend on the concentration ratio of [tex]$\mathrm{Hg}^{2+}$[/tex] and [tex]$\mathrm{I}^{-}$[/tex], the following reactions can happen:
[tex]$\mathrm{Hg}^{2+}+2 \mathrm{I}^{-} \longrightarrow \underset{\text { insoluble red color solid }}{\mathrm{HgI}_2}$[/tex]
If there exists excess [tex]$\mathrm{I}^{-}$[/tex], then this red solid will continue to react with excess [tex]$\mathrm{I}^{-}$[/tex]to form a colorless soluble complex:
[tex]$\mathrm{HgI}_2+\mathrm{I}^{-} \longrightarrow\left[\mathrm{HgI}_3\right]^{-} \\[/tex]
[tex]{\left[\mathrm{HgI}_3\right]^{-}+\mathrm{I}^{-} \longrightarrow\left[\mathrm{HgI}_4\right]^{2-}}\end{gathered}$[/tex]
The metal ion to which the ligands exits attached at the centre of a
coordination complex.
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