NEET Exam > NEET Notes > Short Notes > Short Notes: Coordination Compounds
Short Notes: Coordination Compounds
Basic Terminology
| Term | Definition | Example |
|---|---|---|
| Coordination Entity | Central metal atom/ion + ligands | [CoCl3(NH3)3] |
| Central Atom/Ion | Metal atom/ion to which ligands are attached | Co in [CoCl3(NH3)3] |
| Ligands | Ions/molecules bound to central atom | NH3, Cl- |
| Coordination Number | Number of ligand donor atoms bonded to central atom | 6 in [CoCl3(NH3)3] |
| Coordination Sphere | Central atom + ligands (written in square brackets) | [CoCl3(NH3)3] |
| Oxidation State | Charge on central metal atom if all ligands removed | +3 for Co in above complex |
Types of Ligands
| Based on | Type | Examples |
|---|---|---|
| Denticity | Monodentate (1 donor atom) | Cl-, H2O, NH3, CN- |
| Bidentate (2 donor atoms) | Ethylenediamine (en), oxalate (ox), C2O42- | |
| Polydentate (>2 donor atoms) | EDTA (6 donor atoms) | |
| Charge | Anionic | Cl-, CN-, OH-, SCN- |
| Neutral | H2O, NH3, CO | |
| Special | Ambidentate (can bind through different atoms) | NO2-/ONO-, SCN-/NCS- |
Nomenclature (IUPAC Rules)
- Cationic/Neutral Complex: Name ends with metal name
- Example: [Co(NH3)6]Cl3 = Hexaamminecobalt(III) chloride
- Anionic Complex: Name ends with "-ate"
- Example: K4[Fe(CN)6] = Potassium hexacyanoferrate(II)
- Ligand naming:
- Anionic ligands: end in "-o" (Cl- = chlorido, CN- = cyanido)
- Neutral ligands: unchanged (NH3 = ammine, H2O = aqua)
- Prefixes for number: di-, tri-, tetra-, penta-, hexa-
- For complex ligands: bis-, tris-, tetrakis-
- Order: Alphabetical (ignoring prefixes)
- Oxidation state: Roman numerals in parentheses
Werner's Theory
- Primary Valence: Ionizable, satisfied by anions (oxidation state)
- Secondary Valence: Non-ionizable, satisfied by ligands (coordination number)
- Secondary valencies directed in space → definite geometry
Isomerism in Coordination Compounds
Structural Isomerism
| Type | Description | Example |
|---|---|---|
| Ionization Isomerism | Different ions in solution | [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br |
| Linkage Isomerism | Ambidentate ligand binds through different atoms | [Co(NH3)5NO2]Cl2 and [Co(NH3)5ONO]Cl2 |
| Coordination Isomerism | Interchange of ligands between cationic and anionic entities | [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6] |
| Solvate Isomerism | Solvent molecules as ligands or outside | [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2·H2O |
Stereoisomerism
| Type | Description | Example |
|---|---|---|
| Geometrical Isomerism | Different spatial arrangements
| [Pt(NH3)2Cl2] has cis and trans forms |
| Optical Isomerism | Non-superimposable mirror images (no plane of symmetry) | cis-[Co(en)2Cl2]+ (d and l forms) |
Bonding in Coordination Compounds
Valence Bond Theory (VBT)
- Central metal uses hybridized orbitals to bond with ligands
- Ligands donate electron pairs (coordinate covalent bonds)
- Inner orbital complex: Uses (n-1)d orbitals, low spin, strongly paramagnetic
- Outer orbital complex: Uses nd orbitals, high spin, weakly paramagnetic
| CN | Hybridization | Geometry | Example |
|---|---|---|---|
| 4 | sp3 | Tetrahedral | [NiCl4]2- |
| 4 | dsp2 | Square planar | [Ni(CN)4]2- |
| 5 | sp3d | Trigonal bipyramidal | [Fe(CO)5] |
| 6 | sp3d2 | Octahedral (outer) | [FeF6]3- |
| 6 | d2sp3 | Octahedral (inner) | [Co(NH3)6]3+ |
Crystal Field Theory (CFT)
- Ligands treated as point charges/dipoles
- d-orbitals split in energy due to electrostatic interactions
- Octahedral field:
- d-orbitals split into t2g (dxy, dyz, dzx) - lower energy
- eg (dx²-y², dz²) - higher energy
- Δo = Crystal field splitting energy
- Tetrahedral field:
- e (dx²-y², dz²) - lower energy
- t2 (dxy, dyz, dzx) - higher energy
- Δt = (4/9)Δo
Spectrochemical Series
- Arrangement of ligands based on crystal field splitting ability
- Weak field ligands (small Δ): I- < Br- < SCN- < Cl- < F- < OH- < H2O
- Strong field ligands (large Δ): H2O < NH3 < en < CN- < CO
Magnetic Properties
- Magnetic moment (μ) = √(n(n+2)) BM
- n = number of unpaired electrons
- High spin complex: Weak field ligand, maximum unpaired electrons
- Low spin complex: Strong field ligand, minimum unpaired electrons
Color in Coordination Compounds
- Due to d-d transitions (electron excitation within d-orbitals)
- Energy absorbed = Δ (splitting energy)
- Color observed = Complementary color of absorbed light
- d0 and d10 complexes are colorless (no d-d transitions)
NEET Focus:
- IUPAC nomenclature of coordination compounds
- Calculation of oxidation state and coordination number
- Types of isomerism with examples
- Hybridization and geometry relationship
- Crystal field splitting in octahedral and tetrahedral fields
- Magnetic moment calculations
- Spectrochemical series
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FAQs on Short Notes: Coordination Compounds
| 1. What are coordination compounds? |  |
Ans. Coordination compounds are chemical complexes formed by the coordination of metal ions with molecules or ions known as ligands. These compounds consist of a central metal atom or ion bonded to surrounding ligands, which can be neutral molecules or anions. Their unique structures and bonding characteristics lead to diverse chemical and physical properties.
| 2. What are ligands, and how are they classified? | |
Ans. Ligands are ions or molecules that donate electron pairs to a central metal atom or ion to form coordination complexes. They are classified into two main categories: bidentate ligands, which can form two bonds with the metal centre, and monodentate ligands, which can form only one bond. Other classifications include chelating ligands that form rings with the metal and bridging ligands that link multiple metal centres.
| 3. How does the oxidation state of a metal in a coordination compound affect its properties? | |
Ans. The oxidation state of a metal in a coordination compound significantly influences its reactivity, stability, and colour. Different oxidation states can lead to varying electronic configurations, affecting the compound's ability to participate in redox reactions. Additionally, the oxidation state can alter the geometry of the coordination compound, leading to distinct physical and chemical behaviours.
| 4. What is the significance of the crystal field theory in understanding coordination compounds? | |
Ans. Crystal field theory explains the electronic structure of coordination compounds by considering the effects of the electric field generated by surrounding ligands on the d-orbitals of the metal ion. This theory helps predict the splitting of d-orbitals, which is crucial for understanding the colour, magnetic properties, and stability of coordination compounds based on their geometry and ligand field strength.
| 5. Can you explain the role of coordination compounds in biological systems? | |
Ans. Coordination compounds play vital roles in biological systems, particularly in processes such as oxygen transport and electron transfer. For example, haemoglobin, a coordination complex of iron, binds oxygen in the blood, facilitating its transport. Similarly, many enzymes contain metal ions in their active sites, which are essential for catalysing biochemical reactions.
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