Coordination Compounds: Speed-Solving Techniques (10)

easy • 2 min read
Tags Coordination Compounds

Question

Find the oxidation state, coordination number, and IUPAC name of the central metal in [Co(NH3)4Cl2]Cl[\text{Co}(\text{NH}_3)_4 \text{Cl}_2]\text{Cl}.

Solution — Step by Step

NH3_3 is neutral (charge 0). Cl−^- has charge −1-1. The compound is overall neutral.

The complex ion is [Co(NH3)4Cl2]+[\text{Co}(\text{NH}_3)_4 \text{Cl}_2]^+ (since the outside Cl−^- has charge −1-1, the complex must be +1+1).

Inside the complex: Co + 4(0)4(0) + 2(−1)=+12(-1) = +1 (the charge of the complex). So Co −2=+1⇒- 2 = +1 \Rightarrow Co =+3= +3.

Count ligands attached to Co: 4 NH3_3 + 2 Cl−^- = 66. Coordination number = 6.

Ligands alphabetical (within the complex), prefix multiplicities (di, tri, tetra…). Ligands: ammine (NH3_3, name ‘ammine’), chlorido (Cl−^-, name ‘chlorido’ — modern IUPAC).

Alphabetical: ammine before chlorido.

So: tetraamminedichloridocobalt(III) chloride.

Final answer: Co oxidation state +3+3, coordination number 6, IUPAC name “tetraamminedichloridocobalt(III) chloride”.

Why This Works

For coordination compounds, just count: (charges of ligands) + (oxidation state of metal) = (charge of complex ion). The compound’s overall charge is 0, so opposite-sign ions outside balance the inside.

The IUPAC ligand naming is the trickiest part. Memorise the common ones: ammine (NH3_3), aqua (H2_2O), chlorido (Cl−^-), cyano (CN−^-), hydroxido (OH−^-), oxo (O2−^{2-}).

Alternative Method (Speed)

Use the shortcut formula: oxidation state of M == (overall charge) −- (sum of ligand charges). Here: +1−(4×0+2×−1)=+1−(−2)=+3+1 - (4 \times 0 + 2 \times -1) = +1 - (-2) = +3. One line, no ambiguity.

For naming, count ligands → alphabetise → add prefix → metal name with oxidation state in roman numerals → counter ion. Practice with [Cr(NH3)6]Cl3[\text{Cr}(\text{NH}_3)_6]\text{Cl}_3, K4[Fe(CN)6]\text{K}_4[\text{Fe}(\text{CN})_6], [Co(en)2Cl2]+[\text{Co}(\text{en})_2\text{Cl}_2]^+.

Common Mistake

Forgetting that NH3_3 is neutral but Cl−^- has charge −1-1. Also, the outside Cl is a counter ion, not a ligand. Only ligands inside the brackets count for coordination number.

Old IUPAC vs new IUPAC. Old: ‘chloro’. New (post-2005): ‘chlorido’. Both accepted in JEE/NEET, but recent NCERT uses ‘chlorido’.

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