The central idea
First use the overall charge and ligand charges to find the metal oxidation state. Then count attached donor atoms for the coordination number. Finally distinguish ligands inside square brackets from counterions outside them. These counts answer different questions.
A worked example
In [Co(NH₃)₅Cl]Cl₂, the two external chloride ions imply a +2 complex cation. With neutral NH₃ and one coordinated Cl⁻, cobalt is +3. Five N donors and one Cl donor give coordination number six. Complete dissociation into the complex cation and two counterions gives three ions.
A common mistake
A chloride ligand inside the coordination sphere is not counted as an independently dissociated chloride ion in the simple ionisation model.
Try the idea
These original practice questions also appear in the foundation mock. Attempt them before opening the explanation.
The oxidation state and d-electron count of cobalt in [Co(NH₃)₅Cl]Cl₂ are:
Answer: B
Two external Cl⁻ ions require a complex charge of +2. NH₃ is neutral and the coordinated Cl has charge −1, so x − 1 = +2 and Co is +3. Co is [Ar]3d⁷4s²; removing three electrons gives d⁶.
In the usual crystal-field model, [Fe(CN)₆]⁴⁻ has how many unpaired electrons?
Answer: A
The iron oxidation state is +2, so it is d⁶. CN⁻ is a strong-field ligand and this octahedral complex is low spin. Its six d electrons occupy t₂g⁶, all paired.
An octahedral d⁸ metal ion has two unpaired electrons. Its spin-only magnetic moment is closest to:
Answer: C
Use μ = √[n(n + 2)] BM, where n is the number of unpaired electrons. For n = 2, μ = √8 = 2.83 BM. This is the spin-only estimate.
Which pair consists of linkage isomers?
Answer: A
The ambidentate nitrite ligand can bind through N or through O, giving linkage isomerism. The platinum pair shows geometrical isomerism, the en pair optical isomerism, and the last pair differs in oxidation state.
For octahedral [Co(en)₃]³⁺, where en is neutral ethane-1,2-diamine, which statements are correct?
Answer: A, B, C
Neutral en contributes no charge, so cobalt is +3. Each of the three en ligands binds through two nitrogen atoms, giving coordination number six. There is no coordinated chloride in the formula. Octahedral tris-chelate complexes of this kind can show optical isomerism.
What to do next
Explain your answer aloud, then find a related question in an official paper. If you can only follow a worked solution but cannot start a fresh question, revisit the central idea and try again without the notes.
Apply it to an official paper