The coordination number of aluminum in its common complexes is

$A$ coordination compound $CrCl_{3} \cdot 4 H_{2} O$ precipitates silver chloride when treated with silver nitrate. The molar conductance of its solution corresponds to a total of two ions. Write the structural formula of the compound and name it.

Given the molecular formulas of the hexa-coordinated complexes: $(a)$ $CoCl_3 \cdot 6NH_3$,$(b)$ $CoCl_3 \cdot 5NH_3$,$(c)$ $CoCl_3 \cdot 4NH_3$. If the number of coordinated $NH_3$ molecules in $(a)$,$(b)$,and $(c)$ are $6, 5$,and $4$ respectively,the primary valencies in $(a)$,$(b)$,and $(c)$ are:

Some salts,although containing two different metallic elements,give a test for only one of them in solution. Such salts are:

Primary and Secondary valency of $Co$ in the complex compound $[Co(en)_3]Cl_3$ is respectively . . . . . . .

Identify a ligand with a single donor atom from the following.