Coordination Chemistry
Complex Ions, Ligands, and Coordination Compounds
Key Definitions
Coordination Complex
A central metal atom/ion bonded to surrounding molecules or ions (ligands)
Coordination Number (CN)
Number of ligand donor atoms directly bonded to the central metal
Common CN values: 2, 4, 6
Ligand
A molecule or ion that donates electron pair(s) to the central metal
Monodentate: 1 donor atom (Cl⁻, NH₃, H₂O)
Bidentate: 2 donor atoms (en, ox²⁻)
Polydentate: 3+ donor atoms (EDTA⁴⁻)
Oxidation State
Charge on the central metal atom in the complex
Common Ligands
| Formula | Name | Denticity | Charge |
|---|---|---|---|
| H₂O | Aqua | Monodentate | 0 |
| NH₃ | Ammine | Monodentate | 0 |
| Cl⁻ | Chloro | Monodentate | -1 |
| CN⁻ | Cyano | Monodentate | -1 |
| OH⁻ | Hydroxo | Monodentate | -1 |
| CO | Carbonyl | Monodentate | 0 |
| en | Ethylenediamine | Bidentate | 0 |
| ox²⁻ | Oxalato | Bidentate | -2 |
| EDTA⁴⁻ | Ethylenediaminetetraacetate | Hexadentate | -4 |
Nomenclature Rules
Naming Coordination Compounds
1. Cation before anion (as with all ionic compounds)
2. Ligands in alphabetical order (ignore prefixes like di-, tri-)
3. Use prefixes:
• di-, tri-, tetra-, penta-, hexa- for simple ligands
• bis-, tris-, tetrakis- for complex ligands (e.g., bis(ethylenediamine))
4. Metal name:
• Cationic/neutral complex: use element name
• Anionic complex: use -ate suffix (e.g., ferrate, cuprate)
5. Oxidation state in Roman numerals
Examples
[Co(NH₃)₆]Cl₃
Hexaamminecobalt(III) chloride
K₄[Fe(CN)₆]
Potassium hexacyanoferrate(II)
[Cr(H₂O)₄Cl₂]Cl
Tetraaquadichlorochromium(III) chloride
Crystal Field Theory (CFT)
d-Orbital Splitting
Ligands create an electrostatic field that splits the five d-orbitals into different energy levels
Octahedral (most common)
Higher energy (eg): dx²-y², dz²
Lower energy (t2g): dxy, dxz, dyz
Energy gap: Δo (crystal field splitting energy)
Tetrahedral
Splitting is inverted and smaller: Δt ≈ (4/9)Δo
Spectrochemical Series
Ligands ordered by increasing field strength (Δ):
Weak field: Small Δ, high-spin
Strong field: Large Δ, low-spin
High-Spin vs Low-Spin
For d⁴, d⁵, d⁶, d⁷ octahedral complexes:
High-Spin: Weak field ligands → electrons spread out (Hund's rule)
Low-Spin: Strong field ligands → electrons pair up in lower orbitals
Worked Examples
Example 1: Determine Oxidation State
Problem: What is the oxidation state of Fe in [Fe(CN)₆]³⁻?
Solution:
Let x = oxidation state of Fe
6 CN⁻ ligands each contribute -1 charge
Overall complex charge = -3
x + 6(-1) = -3
x - 6 = -3
x = +3
Fe is in +3 oxidation state
Example 2: Name the Complex
Problem: Name [Pt(NH₃)₂Cl₂]
Solution:
1. Ligands alphabetically: ammine before chloro
2. Count ligands: 2 NH₃ (diammine), 2 Cl⁻ (dichloro)
3. Metal: platinum (neutral complex)
4. Oxidation state: 0 + 2(0) + 2(-1) = -2, so Pt is +2
Diamminedichloroplatinum(II)
Example 3: Coordination Number
Problem: What is the CN of [Co(en)₃]³⁺?
Solution:
en (ethylenediamine) is a bidentate ligand
3 en ligands × 2 donor atoms each = 6 donor atoms
CN = 6
Common Mistakes
Confusing Denticity with Coordination Number
CN counts donor atoms, not ligand molecules!
Forgetting Charge Balance
Sum of oxidation state + ligand charges must equal overall charge!
Wrong -ate Suffix
Only use -ate for anionic complexes (e.g., ferrate, not ferrium)!