Question 1

How is the oxidation number of a metal found in a coordination compound?

Accepted Answer

Treat each ligand as having its known charge, with neutral ligands assigned zero. Add those ligand charges to the unknown oxidation number of the metal; the total must equal the charge of the coordination entity. Counter-ions outside the brackets are not included in that complex-ion equation.

Question 2

Why is the coordination number of [Co(NH3)6]3+ equal to six?

Accepted Answer

Each ammonia ligand coordinates through one nitrogen donor atom. Six NH3 ligands therefore provide six donor atoms bonded to cobalt, giving a coordination number of six and octahedral ligand positions.

Question 3

Are the chloride ions in [Co(NH3)6]Cl3 ligands?

Accepted Answer

No. The six ammonia ligands form the coordination entity [Co(NH3)6]3+. The three chloride ions are outside the coordination sphere and act as counter-ions, making the complete compound electrically neutral.

Question 4

How does ethylenediamine form a chelate ring?

Accepted Answer

Ethylenediamine, written as en, contains two nitrogen donor atoms. Both bind to the same metal ion, and the ligand's carbon chain completes a ring containing the metal, which is called chelation. One en ligand has denticity 2.

Question 5

Why does three en ligands still give coordination number six?

Accepted Answer

Coordination number counts donor atoms rather than ligand molecules. Each en ligand supplies two nitrogen donors, so three en ligands supply 3 × 2 = 6 donor atoms and give [Co(en)3]3+ a coordination number of six.

Build a Complex: Oxidation Number, Coordination Number and Chelation

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