A transition metal ion surrounded by molecules or ions that each donate a lone pair is a complex. Count the dative bonds to get the coordination number, and the coordination number usually tells you the shape.
A complex is a central metal atom or ion surrounded by ligands. A ligand is a species with at least one lone pair that forms a dative covalent (coordinate) bond to the metal. Monodentate ligands (H2O, NH3, Cl−, CN−, OH−) form one bond; bidentate ligands (1,2-diaminoethane, en, and ethanedioate, C2O42−) form two; polydentate EDTA4− forms six. The coordination number is the number of dative bonds to the metal. Coordination number 6 gives an octahedral complex, 4 gives tetrahedral (large ligands such as Cl−) or square planar (Pt2+, as in cisplatin), and 2 gives linear, as in [Ag(NH3)2]+. The overall charge is the metal's oxidation state plus the charges of the ligands.
Each ligand uses a lone pair on a donor atom. Count donor atoms that actually bond to the metal, not atoms in the ligand.
| Ligand | Formula | Charge | Donor atom(s) | Type |
|---|---|---|---|---|
| water | H2O | 0 | O | monodentate |
| ammonia | NH3 | 0 | N | monodentate |
| chloride | Cl− | −1 | Cl | monodentate |
| cyanide | CN− | −1 | C | monodentate |
| hydroxide | OH− | −1 | O | monodentate |
| 1,2-diaminoethane (en) | H2NCH2CH2NH2 | 0 | 2 × N | bidentate |
| ethanedioate | C2O42− | −2 | 2 × O | bidentate |
| EDTA | EDTA4− | −4 | 2 × N, 4 × O | polydentate (hexadentate) |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What type of bond joins a ligand to a central metal ion?
Which ligand is bidentate?
What is the shape of [CuCl4]2−?
What is the coordination number of the metal in [Ag(NH3)2]+?
Use the complex shapes view in the simulator to revise octahedral, tetrahedral, square planar and linear arrangements, with the coordination number and overall charge of each complex.
| Complex | Coordination number | Shape | Metal oxidation state |
|---|---|---|---|
| [Fe(H2O)6]3+ | 6 | octahedral | +3 |
| [Fe(CN)6]4− | 6 | octahedral | +2 (x − 6 = −4) |
| [Cr(C2O4)3]3− | 6 | octahedral | +3 (x − 6 = −3) |
| [CoCl4]2− | 4 | tetrahedral | +2 |
| [Pt(NH3)2Cl2] | 4 | square planar | +2 |
| [Ag(NH3)2]+ | 2 | linear | +1 |
| [Cu(EDTA)]2− | 6 | octahedral | +2 (x − 4 = −2) |
The coordination number counts dative bonds, not ligands: [Cu(en)2(H2O)2]2+ is 6-coordinate. Neutral ligands (H2O, NH3, en) add nothing to the charge, so [Co(NH3)6]3+ contains Co(+3). Four ligands can mean tetrahedral or square planar; Pt(II) complexes are square planar.
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
What is the coordination number of nickel in [Ni(en)3]2+?
What is the oxidation state of chromium in [Cr(C2O4)3]3−?
Which complex is square planar?
A complex contains Fe3+, four water molecules and two hydroxide ions. What is its formula?
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