Coordination Number Calculator
Determine coordination numbers and predict molecular geometries of metal complexes
Use square brackets for the complex, include charge if present
Quick Examples
Coordination Number
Molecular Geometry
Complex Details
Formula:
Central Metal:
Coordination Number:
Ligand Information
Properties and Characteristics
Similar Complexes
Understanding Coordination Number
The coordination number (CN) is the number of ligand donor atoms directly bonded to the central metal ion in a coordination complex. This fundamental concept in coordination chemistry determines the geometry, properties, and reactivity of the complex. The coordination number depends on the size of the metal ion, its charge, and the nature of the ligands.
Key Concepts
- Coordination Number: The total number of donor atoms bonded to the central metal
- Ligand: A molecule or ion that donates electron pairs to the metal
- Denticity: The number of donor atoms in a single ligand (mono-, bi-, tri-, etc.)
- Chelate: A complex with multidentate ligands forming ring structures
- Coordination Sphere: The central metal and its directly attached ligands
Common Coordination Numbers and Geometries
Coordination Number 2
LinearGeometry: Linear (180° bond angle)
Examples: [Ag(NH₃)₂]⁺, [CuCl₂]⁻, [Au(CN)₂]⁻
Coordination Number 4
Tetrahedral or Square PlanarTetrahedral: [CoCl₄]²⁻, [Zn(NH₃)₄]²⁺, [FeCl₄]⁻
Square Planar: [PtCl₄]²⁻, [Ni(CN)₄]²⁻, [AuCl₄]⁻
Note: d⁸ metals (Pt²⁺, Pd²⁺, Au³⁺, Ni²⁺) often adopt square planar geometry
Coordination Number 6
OctahedralGeometry: Octahedral (most common for CN 6)
Examples: [Fe(CN)₆]³⁻, [Co(NH₃)₆]³⁺, [Cr(H₂O)₆]³⁺, [Ni(H₂O)₆]²⁺
Other Coordination Numbers
Various Geometries- • CN 3: Trigonal planar (rare) - [HgI₃]⁻
- • CN 5: Trigonal bipyramidal or Square pyramidal - [Fe(CO)₅]
- • CN 7: Pentagonal bipyramidal - [UO₂F₅]³⁻
- • CN 8: Cubic or Square antiprismatic - [Mo(CN)₈]⁴⁻
- • CN 12: Cuboctahedral - Some lanthanides
Common Ligands and Denticity
Monodentate Ligands
One donor atom per ligand
- • H₂O (aqua)
- • NH₃ (ammine)
- • Cl⁻ (chloro)
- • CN⁻ (cyano)
- • CO (carbonyl)
- • OH⁻ (hydroxo)
Bidentate Ligands
Two donor atoms per ligand
- • en (ethylenediamine)
- • ox²⁻ (oxalate)
- • acac⁻ (acetylacetonate)
- • bipy (bipyridine)
- • phen (phenanthroline)
Polydentate Ligands
Three or more donor atoms
- • dien (tridentate)
- • trien (tetradentate)
- • EDTA⁴⁻ (hexadentate)
- • porphyrin (tetradentate)
Ambidentate Ligands
Can bind through different atoms
- • NO₂⁻ (can bind via N or O)
- • SCN⁻ (can bind via S or N)
- • CN⁻ (can bind via C or N)
Factors Affecting Coordination Number
1. Size of Central Metal Ion
Larger metal ions can accommodate more ligands. Example: CN 6 is more common for larger 3d metals, while smaller metals may prefer CN 4.
2. Charge on Metal Ion
Higher charge increases electrostatic attraction, often leading to higher coordination numbers. Fe³⁺ typically has CN 6, while Fe²⁺ can have CN 4 or 6.
3. Size of Ligands
Bulky ligands lead to lower coordination numbers due to steric hindrance. Small ligands like CN⁻ allow higher coordination numbers.
4. Electronic Configuration
d⁸ metals (Pt²⁺, Pd²⁺, Ni²⁺) strongly prefer square planar geometry with CN 4 due to crystal field stabilization energy.
5. Ligand Denticity
Multidentate ligands (chelating agents) can achieve higher coordination numbers with fewer ligand molecules. EDTA alone can provide CN 6.
Applications of Coordination Chemistry
1. Biological Systems
Hemoglobin contains Fe²⁺ with CN 6 (octahedral), chlorophyll has Mg²⁺ with CN 4 (square planar), and vitamin B₁₂ features Co³⁺ with CN 6.
2. Catalysis
Coordination complexes are widely used as catalysts. Wilkinson's catalyst [RhCl(PPh₃)₃] has CN 4 and catalyzes hydrogenation reactions.
3. Medicine
Cisplatin [PtCl₂(NH₃)₂] with CN 4 (square planar) is a chemotherapy drug. EDTA is used for heavy metal chelation therapy.
4. Materials Science
Coordination polymers and metal-organic frameworks (MOFs) exploit different coordination numbers to create porous materials for gas storage and catalysis.
5. Analytical Chemistry
Coordination complexes are used as indicators (EDTA titrations), in colorimetric analysis, and for selective metal ion detection.
References
The coordination chemistry principles and geometries are based on fundamental inorganic chemistry from reputable sources:
Note: This calculator determines coordination numbers based on standard ligand denticities. For complex cases with bridging ligands, unusual bonding modes, or organometallic compounds, consult advanced coordination chemistry references. Geometry predictions are based on common patterns but may vary with specific electronic configurations and steric effects.
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