What are Coordination Compounds?
Coordination compounds (also called coordination complexes) are molecules or ions that contain a central metal atom or ion bonded to one or more ligands (molecules or ions) through coordinate covalent bonds. These compounds play crucial roles in biological systems, industrial catalysis, and analytical chemistry.
Historical Background
The systematic study of coordination compounds began with Alfred Werner in 1893. His revolutionary coordination theory earned him the Nobel Prize in Chemistry in 1913. Werner proposed that metal atoms can exhibit two types of valence:
- Primary valence: The oxidation state of the metal (satisfied by anions)
- Secondary valence: The coordination number (number of ligands directly bonded)
Components of Coordination Compounds
Central Metal Atom/Ion
Lewis Acid
Usually a transition metal ion (d-block elements)
Acts as an electron pair acceptor
Examples: Fe²⁺, Fe³⁺, Co³⁺, Cu²⁺, Ni²⁺, Pt²⁺, Cr³⁺
Can also be main group metals: Al³⁺, Zn²⁺
Ligands
Lewis Bases
Molecules or ions with lone pairs of electrons
Act as electron pair donors
Common examples: H₂O, NH₃, Cl⁻, CN⁻, CO, en (ethylenediamine)
Form coordinate covalent bonds with the metal
Coordination Sphere
The Complex Ion
Central metal plus directly attached ligands
Written in square brackets: [MLn]charge
Example: [Co(NH₃)₆]³⁺
Overall charge = metal charge + ligand charges
Types of Ligands
Based on Denticity (Number of Donor Atoms)
Monodentate
One donor atom per ligand
Examples:
- H₂O (aqua)
- NH₃ (ammine)
- Cl⁻ (chloro)
- CN⁻ (cyano)
- CO (carbonyl)
Bidentate
Two donor atoms per ligand
Examples:
- en (ethylenediamine): H₂N-CH₂-CH₂-NH₂
- ox²⁻ (oxalate): C₂O₄²⁻
- acac⁻ (acetylacetonate)
- bipy (bipyridine)
Polydentate
Three or more donor atoms
Examples:
- EDTA⁴⁻ (hexadentate)
- dien (tridentate)
- porphyrin (tetradentate)
- Crown ethers
Ambidentate
Can bind through different atoms
Examples:
- SCN⁻: S-bonded (thiocyanato) or N-bonded (isothiocyanato)
- NO₂⁻: N-bonded (nitro) or O-bonded (nitrito)
- CN⁻: C-bonded or N-bonded
Based on Charge
Anionic ligands: Cl⁻, Br⁻, I⁻, OH⁻, CN⁻, SCN⁻, NO₂⁻, CO₃²⁻, SO₄²⁻, ox²⁻
Neutral ligands: H₂O, NH₃, CO, NO, py (pyridine), en, bipy
Cationic ligands: NO⁺, NO₂⁺ (rare)
Coordination Number and Geometry
The coordination number (CN) is the number of ligand donor atoms directly bonded to the central metal ion.
| Coordination Number | Common Geometries | Examples |
|---|---|---|
| 2 | Linear | [Ag(NH₃)₂]⁺, [CuCl₂]⁻ |
| 4 | Tetrahedral, Square Planar | [Ni(CO)₄] (Td), [PtCl₄]²⁻ (SP), [Ni(CN)₄]²⁻ (SP) |
| 5 | Trigonal Bipyramidal, Square Pyramidal | [Fe(CO)₅], [Ni(CN)₅]³⁻ |
| 6 | Octahedral | [Co(NH₃)₆]³⁺, [Fe(CN)₆]⁴⁻, [Cr(H₂O)₆]³⁺ |
| 7 | Pentagonal Bipyramidal | [ZrF₇]³⁻ |
| 8 | Square Antiprismatic, Cubic | [Mo(CN)₈]⁴⁻ |
Factors Affecting Coordination Number:
- Size of the central metal ion (larger ions accommodate more ligands)
- Size of the ligands (bulky ligands reduce CN)
- Electronic configuration of the metal
- Steric effects and ligand-ligand repulsion
Nomenclature of Coordination Compounds
The International Union of Pure and Applied Chemistry (IUPAC) provides systematic rules for naming coordination compounds:
General Rules:
- Cation is named first, then anion
- Ligands are named in alphabetical order (prefixes ignored)
- Numerical prefixes: di-, tri-, tetra-, penta-, hexa-
- For polydentate ligands: bis-, tris-, tetrakis-
- Anionic ligands end in "-o" (chloro, cyano, hydroxo)
- Neutral ligands keep their name (exceptions: aqua for H₂O, ammine for NH₃)
- Metal name followed by oxidation state in Roman numerals
- If the complex is an anion, metal name ends in "-ate"
Common Ligand Names
| Formula | Name | Formula | Name |
|---|---|---|---|
| H₂O | aqua | NH₃ | ammine |
| Cl⁻ | chloro | Br⁻ | bromo |
| CN⁻ | cyano | OH⁻ | hydroxo |
| CO | carbonyl | NO₂⁻ | nitro (N-bonded) |
| ONO⁻ | nitrito (O-bonded) | SCN⁻ | thiocyanato |
| en | ethylenediamine | ox²⁻ | oxalato |
| EDTA⁴⁻ | ethylenediaminetetraacetato | py | pyridine |
Naming Examples
[Co(NH₃)₆]Cl₃
Hexaamminecobalt(III) chloride
6 NH₃ ligands + Co in +3 oxidation state + 3 Cl⁻ counterions
K₄[Fe(CN)₆]
Potassium hexacyanoferrate(II)
Complex anion: metal name ends in "-ate"
[Cr(H₂O)₄Cl₂]Cl
Tetraaquadichlorochromium(III) chloride
Ligands in alphabetical order: aqua before chloro
[Pt(NH₃)₂Cl₂]
Diamminedichloroplatinum(II) or cisplatin (common name)
Neutral complex: no counterions
[Co(en)₃]Cl₃
Tris(ethylenediamine)cobalt(III) chloride
Uses "tris" for polydentate ligands to avoid ambiguity
Isomerism in Coordination Compounds
Coordination compounds exhibit various types of isomerism, making them structurally diverse and interesting:
Structural Isomerism
Ionization Isomerism
Different ions in solution due to exchange between coordinated and non-coordinated groups
Example:
[Co(NH₃)₅SO₄]Br (gives Br⁻ in solution)
[Co(NH₃)₅Br]SO₄ (gives SO₄²⁻ in solution)
Linkage Isomerism
Ambidentate ligands bonding through different atoms
Example:
[Co(NH₃)₅(NO₂)]²⁺ (nitro, N-bonded)
[Co(NH₃)₅(ONO)]²⁺ (nitrito, O-bonded)
Coordination Isomerism
Exchange of ligands between cationic and anionic complexes
Example:
[Co(NH₃)₆][Cr(CN)₆]
[Cr(NH₃)₆][Co(CN)₆]
Hydrate Isomerism
Different positions of water molecules
Example:
[Cr(H₂O)₆]Cl₃ (violet)
[Cr(H₂O)₅Cl]Cl₂·H₂O (blue-green)
[Cr(H₂O)₄Cl₂]Cl·2H₂O (dark green)
Stereoisomerism
Geometrical Isomerism
Cis-Trans (Square Planar & Octahedral)
Square planar MA₂B₂: cis and trans
Example: [Pt(NH₃)₂Cl₂]
Octahedral MA₄B₂: cis and trans
Example: [Co(NH₃)₄Cl₂]⁺
Fac-Mer (Octahedral MA₃B₃)
Example: [Co(NH₃)₃(NO₂)₃]
Optical Isomerism
Non-superimposable mirror images (enantiomers)
Chiral complexes rotate plane-polarized light
Examples:
[Co(en)₃]³⁺ (Δ and Λ forms)
cis-[Co(en)₂Cl₂]⁺
[Cr(ox)₃]³⁻
Important in asymmetric catalysis and biochemistry
Bonding Theories in Coordination Compounds
Several theories explain the bonding in coordination compounds:
Valence Bond Theory (VBT)
Metal uses hybridized orbitals to accommodate ligand electrons
Explains geometry and magnetism
Inner orbital (low spin) vs. outer orbital (high spin) complexes
Crystal Field Theory (CFT)
Electrostatic interaction between metal and ligands
d-orbital splitting explains color and magnetism
High spin vs. low spin based on Δ and pairing energy
Molecular Orbital Theory
Most comprehensive bonding description
Considers both σ and π bonding
Explains spectrochemical series
Predicts bonding, antibonding, and non-bonding MOs
Stability of Coordination Compounds
Thermodynamic Stability
Measured by stability constants (formation constants, Kf) or stability constant (β):
Kf = [MLn] / ([M][L]ⁿ)
Larger Kf = more stable complex
EDTA complexes typically have very high Kf values (chelate effect)
The Chelate Effect
- Chelating ligands (polydentate) form more stable complexes than monodentate ligands
- Entropic advantage: fewer particles → greater entropy increase
- Ring formation provides additional stability
- Five and six-membered rings are most stable
- EDTA⁴⁻ is an excellent chelating agent (hexadentate)
Chelate Effect Example:
[Ni(H₂O)₆]²⁺ + 6NH₃ ⇌ [Ni(NH₃)₆]²⁺ + 6H₂O
vs.
[Ni(H₂O)₆]²⁺ + 3en ⇌ [Ni(en)₃]²⁺ + 6H₂O
The ethylenediamine complex is more stable despite similar donor atoms!
Applications of Coordination Compounds
Biological Systems
Hemoglobin: Fe²⁺-porphyrin complex for oxygen transport
Chlorophyll: Mg²⁺-porphyrin for photosynthesis
Vitamin B₁₂: Co³⁺ complex
Enzymes: Metalloenzymes with metal cofactors
Medicine
Cisplatin: [Pt(NH₃)₂Cl₂] anticancer drug
EDTA: Chelation therapy for heavy metal poisoning
Gold complexes: Arthritis treatment
Contrast agents: MRI imaging (Gd³⁺ complexes)
Industrial Catalysis
Wilkinson's catalyst: [RhCl(PPh₃)₃] for hydrogenation
Ziegler-Natta: Polymerization catalysts
Homogeneous catalysis: Fine chemical synthesis
Organometallics: C-C bond formation
Analytical Chemistry
EDTA titrations: Water hardness determination
Colorimetric detection: Metal ion identification
Complexometric titrations: Quantitative analysis
Extraction: Separation of metal ions
Metallurgy
Extraction of metals: Au, Ag using CN⁻
Purification: Mond process for Ni (using CO)
Electroplating: Metal deposition from complex baths
Photography
Silver complexes: [Ag(S₂O₃)₂]³⁻ in film fixing
Soluble complexes remove unexposed silver halides
Materials Science
Pigments and dyes: Prussian blue Fe₄[Fe(CN)₆]₃
Luminescent materials: Lanthanide complexes
Molecular magnets: Spin-crossover materials
Water Treatment
Sequestration: Preventing metal ion precipitation
EDTA: Softening hard water
Coagulation: Removal of contaminants
Important Coordination Compounds
Prussian Blue - Fe₄[Fe(CN)₆]₃
Deep blue pigment used in paints and blueprints
Antidote for thallium and radioactive cesium poisoning
Cisplatin - cis-[Pt(NH₃)₂Cl₂]
First platinum-based chemotherapy drug
Binds to DNA and prevents cell division in cancer cells
Ferrocene - [Fe(C₅H₅)₂]
Sandwich compound with Fe²⁺ between two cyclopentadienyl rings
Pioneering organometallic compound (Nobel Prize 1973)
Wilkinson's Catalyst - [RhCl(PPh₃)₃]
Homogeneous catalyst for alkene hydrogenation
Nobel Prize-winning work (1973)
Vitamin B₁₂ - Cobalamin
Essential nutrient with Co³⁺ coordinated to corrin ring
Only vitamin containing a metal ion
Study Tips for Coordination Chemistry
- Master IUPAC nomenclature rules - practice naming many compounds
- Understand the relationship between structure and properties (color, magnetism)
- Be able to determine oxidation states and coordination numbers
- Know common ligands, their names, and whether they're mono- or polydentate
- Practice drawing structures showing isomerism (cis/trans, fac/mer, optical)
- Understand the bonding theories (VBT, CFT) and when each is most useful
- Recognize biologically and industrially important coordination compounds
- Learn the spectrochemical series for predicting high/low spin
Related Topics to Explore
- Valence Bond Theory: Hybridization in coordination compounds
- Crystal Field Theory: d-orbital splitting and CFSE calculations
- Molecular Orbital Theory: σ and π bonding in complexes
- Organometallic Chemistry: Metal-carbon bonds
- Bioinorganic Chemistry: Metal ions in biological systems
- Catalysis: Homogeneous and heterogeneous catalytic processes
- Spectroscopy: UV-Vis, IR, and NMR of coordination compounds
- Reaction Mechanisms: Substitution and electron transfer reactions