Transition metals form complex ions in which a central metal cation is surrounded by species called ligands. The bonds that hold these structures together are coordinate (dative covalent) bonds, which determine the shape and coordination number of the complex.
A ligand is a molecule or ion that donates a lone pair of electrons to a central metal ion to form a coordinate bond.
The coordination number is the total number of coordinate bonds formed between ligands and the central metal ion.
A monodentate ligand is a ligand that forms only one coordinate bond with the central metal ion by donating one lone pair of electrons (e.g. \(\text{H}_2\text{O}\), \(\text{NH}_3\), \(\text{Cl}^-\), \(\text{OH}^-\)).
A bidentate ligand is a ligand that forms two coordinate bonds with the central metal ion by donating two lone pairs of electrons from two different atoms in the same molecule (e.g. 1,2-diaminoethane and ethanedioate).
Classification of Ligands
Ligands are classified by the number of coordinate bonds they can form simultaneously with a single metal ion:
- Monodentate ligands: These are relatively small molecules or ions. Water (\(\text{H}_2\text{O}\)) and ammonia (\(\text{NH}_3\)) are neutral monodentate ligands. Chloride (\(\text{Cl}^-\)), cyanide (\(\text{CN}^-\)), and hydroxide (\(\text{OH}^-\)) are negatively charged monodentate ligands.
- Bidentate ligands: These possess two donor atoms, each containing a lone pair.
- 1,2-diaminoethane (en): A neutral ligand with the formula \(\text{H}_2\text{N-CH}_2\text{-CH}_2\text{-NH}_2\). Both nitrogen atoms donate a lone pair.
- Ethanedioate (\(\text{C}_2\text{O}_4^{2-}\)): A negatively charged ligand where two oxygen atoms donate lone pairs.
- Hexadentate ligands: These possess six donor atoms. The most common example is EDTA4- (ethylenediaminetetraacetate). A single \(\text{EDTA}^{4-}\) ligand can form six coordinate bonds (using two nitrogen lone pairs and four carboxylate oxygen lone pairs), completely wrapping around a metal ion to form a highly stable octahedral complex.
Calculating Complex Ion Charges
The overall charge on a complex ion is the sum of the oxidation state of the central metal ion and the charges of the ligands. Water and ammonia carry no charge, whereas chloride (\(\text{Cl}^-\)) and hydroxide (\(\text{OH}^-\)) have a charge of -1.
🔑 Key Principle: Overall Charge Calculation
The calculation is simple: \[\text{Overall Charge} = \text{Metal Oxidation State} + \sum(\text{Ligand Charges})\] Examples:
- \([\text{Fe}(\text{H}_2\text{O})_6]^{3+}\): Iron is in the +3 oxidation state. Water is neutral. Charge = +3 + 6(0) = +3.
- \([\text{CuCl}_4]^{2-}\): Copper is in the +2 oxidation state. Each chloride is -1. Charge = +2 + 4(-1) = -2.
- \([\text{Fe}(\text{CN})_6]^{4-}\): Cyanide is -1. If overall charge is -4, iron oxidation state is: \(x + 6(-1) = -4 \Rightarrow x = +2\).
Always write complex formulas in square brackets with the overall charge superscripted outside the bracket, for example: \([\text{Co}(\text{NH}_3)_6]^{3+}\). Inside the bracket, write the metal symbol first, followed by neutral ligands, then charged ligands.
Shapes of Complexes
The coordination number determines the geometry of the coordinate bonds. You must know these four common shapes, their bond angles, and key examples:
| Coordination Number | Shape | Bond Angle(s) | Examples |
|---|---|---|---|
| 6 | Octahedral | 90° | \([\text{Cu}(\text{H}_2\text{O})_6]^{2+}\), \([\text{Fe}(\text{H}_2\text{O})_6]^{3+}\), \([\text{Co}(\text{NH}_3)_6]^{3+}\) |
| 4 | Tetrahedral | 109.5° | \([\text{CuCl}_4]^{2-}\), \([\text{CoCl}_4]^{2-}\) |
| 4 | Square Planar | 90° | \([\text{Pt}(\text{NH}_3)_2\text{Cl}_2]\) (Cisplatin) |
| 2 | Linear | 180° | \([\text{Ag}(\text{NH}_3)_2]^+\) (Tollens' reagent) |
🔑 Key Principle: Why Chloride Forms Tetrahedral Complexes
Water and ammonia are relatively small ligands, so six of them can easily pack around a Period 4 metal ion, forming 6 coordinate bonds (octahedral). Chloride (\(\text{Cl}^-\)) ligands are larger and carry a negative charge, which creates significant electrostatic repulsion. Therefore, only four chloride ligands can fit around the metal ion, resulting in a coordination number of 4 and a tetrahedral geometry.
Isomerism in Transition Metal Complexes
Transition metal complexes can exhibit stereoisomerism: molecules with the same structural formula but a different 3D spatial arrangement. The two main types are geometric (cis-trans) isomerism and optical isomerism.
Geometric (Cis-Trans) Isomerism
Geometric isomerism occurs in square planar and octahedral complexes when ligands are arranged differently in space.
1. Square Planar Complexes
Cisplatin is the neutral complex [Pt(NH₃)₂Cl₂]. It exhibits square planar geometry around the platinum(II) ion. Because it has two different types of ligands, cisplatin can exist as geometric isomers: cisplatin (the cis isomer, where both chlorine ligands are adjacent, at a 90° bond angle) and transplatin (the trans isomer, where the chlorines are opposite each other, at a 180° bond angle).
Only the cis isomer, cisplatin, is biologically active and used as an anticancer drug. It functions by entering cancer cells and undergoing ligand substitution, where the chloride ligands are replaced by nitrogen donor atoms on DNA bases (specifically guanine). This binds cisplatin to DNA, forming cross-links that distort the DNA double helix. This distortion prevents DNA replication and transcription, triggering programmed cell death (apoptosis) in rapidly dividing cancer cells.
You must specify that cisplatin has a square planar geometry. A common error is assuming that CN = 4 implies a tetrahedral geometry. Make sure you can draw cisplatin showing Pt at the centre and all four ligands in a square planar configuration with 90° bond angles.
2. Octahedral Complexes
Geometric isomerism also occurs in octahedral complexes containing four monodentate ligands of one type and two of another (e.g. [Co(NH₃)₄Cl₂]⁺). In the cis isomer, the two identical ligands (the chlorides) are adjacent to each other at 90°. In the trans isomer, they are opposite each other at 180°.
Optical Isomerism
Optical isomerism occurs in octahedral complexes, most commonly those containing bidentate ligands. Optical isomers are non-superimposable mirror images of each other, known as enantiomers. They are chemically identical but rotate plane-polarised light in opposite directions.
You need to know these key examples of optical isomerism in transition metal complexes:
- [Co(en)₃]³⁺: An octahedral complex containing three neutral bidentate 1,2-diaminoethane (en) ligands. It has no plane of symmetry and forms a pair of enantiomers.
- [Fe(C₂O₄)₃]³⁻: An octahedral complex containing three bidentate ethanedioate (oxalate) ligands, forming non-superimposable mirror images.
- cis-[Co(en)₂Cl₂]⁺: The cis geometric isomer of this complex is chiral and exhibits optical isomerism because the two chloride ligands are adjacent. The corresponding trans isomer has a plane of symmetry (with the chlorides opposite each other) and is therefore optically inactive.
Optical isomers (enantiomers) are stereoisomers that are non-superimposable mirror images of each other and rotate plane-polarised light in opposite directions.
Step 1: Determine the coordination number
Ammonia (\(\text{NH}_3\)) is a monodentate ligand, meaning each ammonia molecule forms one coordinate bond. Since there are six ammonia molecules, the total number of coordinate bonds is 6. Thus, the coordination number is 6.
Step 2: Determine the shape
A coordination number of 6 corresponds to an octahedral shape.
Step 3: State the overall charge
The cobalt ion is in the +3 oxidation state (\(\text{Co}^{3+}\)), and ammonia is neutral. The overall charge is (+3) + 6(0) = +3. The complex is written as \([\text{Co}(\text{NH}_3)_6]^{3+}\).
Step 1: Set up the algebraic equation
Let \(x\) represent the oxidation state of iron. Cyanide (\(\text{CN}^-\)) is a monodentate ligand carrying a -1 charge. The complex has six cyanide ligands and an overall charge of -4.
\[x + 6(-1) = -4\]
Step 2: Solve for \(x\)
\[x - 6 = -4 \Rightarrow x = +2\]
Therefore, the oxidation state of iron in this complex is +2.
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