In the complex, ${\mathrm{CoCl}}_3. 4{\mathrm{NH}}_3$, the primary valency of the central metal is satisfied by

According to Werner's theory, primary valency of a metal is satisfied by negative ions only.

Werner formula …${\mathrm{CoCl}}_3. 4{\mathrm{NH}}_3$

Modern formula . . .$\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_4{\mathrm{Cl}}_2\right]\mathrm{Cl}$

Two chloride ions satisfy both primary and secondary valency. One chloride ion satisfies only primary valency.