An octahedral complex having molecular composition ${\rm{Co}}.5{\rm{N}}{{\rm{H}}_3} \cdot {\rm{C}}{{\rm{l}}^2}.{\rm{S}}{{\rm{O}}_4}$ has two isomers A and B. The solution of A gives a white precipitate with ${\rm{AgN}}{{\rm{O}}_3}$ solution and the solution of B gives white precipitate with ${\rm{BaC}}{{\rm{l}}_2}$ solution. The type of isomerism exhibited by the complex is.