The coordination number for the compound [Co NH3 6]Cl3 is 6. This is because there are six ammonia NH3 ligands surrounding the central cobalt Co atom, forming a complex ion.This compound can exhibit geometrical isomerism, specifically, optical isomerism. Optical isomers are non-superimposable mirror images of each other, also known as enantiomers. In the case of [Co NH3 6]Cl3, the arrangement of the six ammonia ligands around the cobalt atom can form two different enantiomers, which are optically active and can rotate plane-polarized light in opposite directions.