Chemistry · Coordination Compounds · NEET
Inside the square brackets you write the central metal atom and all its directly attached ligands. This whole unit is the coordination sphere and it behaves as ONE single ion (or one neutral unit). Anything written outside the brackets is the counter ion. Example: in [Co(NH3)6]Cl3, the part [Co(NH3)6] is inside (it is the complex ion) and the 3 Cl- are outside (counter ions). In water only the outside Cl- break away.
A ligand is directly bonded to the central metal using a coordinate bond, so it sits INSIDE the brackets and does not leave the metal in solution. A counter ion is NOT bonded to the metal; it only balances the charge of the complex ion and sits OUTSIDE the brackets, so it separates freely in water. The same species, like Cl-, can act as a ligand in one place and a counter ion in another. Position (inside vs outside the brackets) tells you which role it plays.
Only the chloride ions OUTSIDE the coordination sphere (the counter ions) are free in water, so only they react with AgNO3 to give a white AgCl precipitate at once. Chloride that is a ligand INSIDE the brackets is tightly bonded to the metal and does NOT precipitate. So counting the AgCl formed tells you how many Cl- are counter ions.
The central atom is the metal ion in the middle (usually a d-block metal like Co, Fe, Ni, Cu, Cr). It accepts electron pairs. The ligands are the ions or molecules around it (NH3, H2O, Cl-, CN-, en) that DONATE a lone pair to the metal. Simple rule: metal in the centre = central atom, the groups touching it = ligands.
Yes. The charge on the coordination sphere = (oxidation number of the metal) + (sum of the charges of all ligands inside the brackets). The counter ions outside must exactly cancel this charge. Example: in [Co(NH3)4(H2O)Cl]Cl2, Co is +3, one Cl- ligand is -1, NH3 and H2O are neutral, so the sphere is +3-1 = +2, which needs two Cl- counter ions outside. This is the fastest way to write or check a formula.
The correct order of the stoichiometries of AgCl formed when AgNO3 in excess is treated with the complexes CoCl3.6NH3, CoCl3.5NH3, CoCl3.4NH3 respectively is:
A 1:3 electrolyte in aqueous solution is:
The formula of tetraammineaquachloridocobalt(III) chloride is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. The counter ion is not bonded to the metal and is not part of the complex ion. It sits outside the square brackets only to balance charge, and it separates freely when the compound dissolves in water.
Yes. For example Cl- is a ligand inside [Co(NH3)5Cl]2+ but a counter ion outside in [Co(NH3)5Cl]Cl2. What decides its role is whether it is written inside or outside the brackets, that is, whether it is bonded to the metal.
The coordination sphere is the central metal atom plus all its ligands, written together inside square brackets, and it behaves as a single ion in solution.
Electrical conductance depends on the number of free ions in solution. Only counter ions (outside the brackets) break away and move freely, so they carry the current. Ligands stay bonded inside the sphere and do not move independently.
They mean almost the same in NEET. The metal in the middle is usually a positive ion (like Co3+), so it is often called the central metal ion or central atom. Both terms point to the same metal accepting lone pairs from ligands.
