Definition: Coordination Compounds
Coordination compounds are those molecular compounds which retain their identity in solid as well as in aqueous solution. In these compounds, metals or atoms are bonded to a number of anions or neutral molecules by a coordinate bond.
Definition: Coordination Compounds
Coordination compounds are compounds in which a central metal atom or ion is linked to a number of ions or neutral molecules by coordinate bonds — i.e., by donation of lone pair of electrons by these ions or neutral molecules (called ligands) to the central metal atom.
Define monodentate ligand.
A monodentate ligand is one in which a single donor atom shares an electron pair with the centre metal ion to create a coordinate bond.
In the coordination compound, the species surrounding the central metal atom or ion are called ligands.
Definition: Ligand
A ligand is a molecule, ion or group that is bonded to the metal atom or ion in a complex or coordination compound by a coordinate bond.
Define coordination number.
Coordination number is the number of ligand donor atoms directly bonded to the central metal atom or ion in a complex.
Example: In [Co(NH3)5Cl]2+, the coordination number of cobalt (Co) is 6 because 5 ammonia molecules and 1 chloride ion are attached to it.
Define Anionic sphere complex.
A negatively charged coordination sphere or a coordination compound having a negatively charged coordination sphere is called anionic complex or anionic sphere complex.
Define coordination sphere. Give example.
The central metal ion and ligands linked to it are enclosed in a square bracket. This is called a coordination sphere. This is a discrete structural unit. The ionisable groups shown outside the bracket are the counter ions. For example, the compound K4[Fe(CN)6] has [Fe(CN)6]4- coordination sphere with the ionisable K⊕ ions representing counter ions.
Definition: Effective Atomic Number (EAN) Rule
The total number of electrons present on the central metal atom/ion, including those gained by it in bonding, is called EAN.
Define the term Hydrated isomers.
Isomers in which there is exchange of solvent (water) ligands between coordination and ionization spheres are called hydrate isomers.
Two or more coordination compounds which contain the same number and types of atoms, and bonds (i.e., the connectivity between atoms is the same), but which have different spatial arrangements of the atoms are called distereoisomers.
Define the term Co-ordination isomer.
Isomers which show interchange of ligands between cationic and anionic spheres of different metal ions are called co-ordination isomers.
Definition: Cis-isomer
In a disubstituted complex molecule/ion, when two same ligands are at right angles (90°), the geometrical isomer is known as a cis-isomer.
Definition: Trans-isomer
When two ligands are in opposite directions, i.e., at 180° to each other, the isomer formed is the trans-isomer.
Definition: Enantiomers
Optical isomers are mirror images that cannot be superimposed on one another. These are called enantiomers.
Definition: Isomerism
Isomerism is the phenomenon in which compounds have the same molecular formula but differ in their physical or chemical properties due to a different arrangement of atoms or groups in space or structure.
Definition: Weak field ligands
Ligands that cause small splitting of d-orbitals and do not cause pairing of electrons are called weak field ligands.
Definition: Effective atomic number (EAN)
The total number of electrons surrounding the central metal ion after complex formation is called the effective atomic number (EAN).
EAN = number of electrons of metal ion + total number of electrons donated by ligands
= atomic number of metal (Z) - number of electrons lost by metal to form the ion (X) + number of electrons donated by ligands (Y).
= Z - X + Y
Definition: Coordination compound
A compound in which a central metal atom or ion is surrounded by ligands bonded through coordinate bonds is called a coordination compound.
Definition: Ligand
An ion or molecule that donates a pair of electrons to the central metal atom or ion to form a coordinate bond is called a ligand.
Definition: Monodentate ligand
A ligand that contains only one donor atom and forms only one coordinate bond with the central metal ion is called a monodentate ligand.
Definition: Ambidentate ligand
A ligand that can coordinate to the metal through two different donor atoms but only one at a time is called an ambidentate ligand.
Definition: Coordination number
The number of ligand donor atoms directly bonded to the central metal ion in a complex is called the coordination number.
Definition: Coordination complex
A compound that retains its identity in solution and does not completely dissociate into simple ions is called a coordination complex.
Definition: Anionic complex
A complex having a negatively charged coordination sphere is called an anionic complex.
Definition: Neutral complex
A complex having no charge on the coordination sphere is called a neutral complex.
Definition: Linkage isomerism
Isomerism arising due to different donor atoms of the same ligand is called linkage isomerism.
Definition: Coordination isomerism
Isomerism arising due to interchange of ligands between cationic and anionic complexes is called coordination isomerism.
Definition: Solvate (hydrate) isomerism
Isomerism arising due to the presence of solvent molecules inside or outside the coordination sphere is called solvate (hydrate) isomerism.
Definition: Geometrical isomerism
Isomerism in which ligands differ in their spatial arrangement around the central metal ion is called geometrical isomerism.
Definition: Optical isomerism
Isomerism in which two compounds are non-superimposable mirror images of each other is called optical isomerism.
Definition: Chelation
The removal of metal ions from solution by forming stable coordination complexes is called chelation.
Definition: Electroplating
The process of depositing a thin layer of metal over another material using coordination compounds is called electroplating.
Definition: Ionization isomerism
Isomerism arising due to exchange of ligands between coordination sphere and ionization sphere is called ionization isomerism.
Definition: Polydentate ligand
A ligand that contains two or more donor atoms and forms multiple coordinate bonds with the central metal ion is called a polydentate ligand.
Definition: Homoleptic complex
A complex containing only one type of ligand attached to the central metal ion is called a homoleptic complex.
Definition: Heteroleptic complex
A complex containing more than one type of ligand attached to the central metal ion is called a heteroleptic complex.
Definition: Cationic complex
A complex having a positively charged coordination sphere is called a cationic complex.
Definition: Strong field ligands
Ligands that cause large splitting of d-orbitals and pairing of electrons are called strong field ligands.
Key Points: Types of Ligands
| Type of Ligand |
Number of Donor Atoms |
Description |
Examples |
| Monodentate |
1 |
Binds through one donor atom |
Cl⁻, OH⁻, CN⁻, NH₃ |
| Bidentate |
2 |
Binds through two donor atoms |
Ethylenediamine |
| Polydentate |
≥2 |
Binds through two or more donor atoms |
EDTA⁴⁻ (general category) |
| Hexadentate |
6 |
Binds through six donor atoms |
EDTA⁴⁻ |
| Ambidentate |
2 (one at a time) |
Has two donor atoms but uses only one to bind |
CN⁻, SCN⁻, NCS⁻ |
Key Points: Terms Used in Coordination Chemistry
Basic Terms:
- Coordination sphere: Metal + ligands inside brackets
- Charge number: Net charge on complex ion
- Oxidation state: Charge on central metal ion
- Coordination number: Number of donor atoms attached to metal
Double Salts vs Coordination Compounds:
| Property |
Double Salt |
Coordination Compound |
| Dissociation |
Completely into ions |
Gives complex ion |
| Example |
Mohr’s salt |
K₄[Fe(CN)₆] |
Key Points: Classification of Complexes
1. On the basis of ligands
- Homoleptic complex:
Contains only one type of ligand
Example: [Co(NH₃)₆]³⁺
- Heteroleptic complex:
Contains two or more types of ligands
Example: [Co(NH₃)₄Cl₂]⁺
2. On the basis of charge
- Cationic complex:
Positively charged coordination sphere
Example: [Zn(NH₃)₄]²⁺
- Anionic complex:
Negatively charged coordination sphere
Example: [Fe(CN)₆]³⁻
- Neutral complex:
No overall charge
Example: [Ni(CO)₄]
Key Points: IUPAC Nomenclature of Coordination Compounds
1. Order
- Ligand → Metal
- Cation first, then anion
2. Ligand Names
- Cl⁻ → chloro
- CN⁻ → cyano
- OH⁻ → hydroxo
- NH₃ → ammine
- H₂O → aqua
- CO → carbonyl
3. Number Prefix
- di, tri, tetra, penta, hexa
- Special: bis, tris (if ligand has number)
4. Order of Ligands
5. Metal Name
- Neutral/cation → normal name
- Anionic complex → ends with “-ate”
Fe → ferrate
Cu → cuprate
Co → cobaltate
6. Oxidation State
7. Important
- Counter ions not named
- Complex always in [ ]
8. Examples
Neutral complexes
- [Co(NO₂)₃(NH₃)₃] → Triamminetrinitrocobalt(III)
- Fe(CO)₅ → Pentacarbonyliron(0)
Cationic complexes
- [Cu(NH₃)₄]²⁺ → Tetraamminecopper(II) ion
- [Fe(H₂O)₅(NCS)]²⁺ → Pentaaquathiocyanatoiron(III) ion
Anionic complexes
- [Ni(CN)₄]²⁻ → Tetracyanonickelate(II) ion
- [Fe(CN)₆]⁴⁻ → Hexacyanoferrate(II) ion
Compounds (Very Important)
- [Co(NH₃)₅Cl]Cl₂ → Pentaamminechlorocobalt(III) chloride
- K₃[Al(C₂O₄)₃] → Potassium trioxalatoaluminate(III)
- Na₃[Co(NO₂)₆] → Sodium hexanitrocobaltate(III)
Key Points: Nomenclature of Coordination Compounds
Rules for Writing Formulae:
- The cation is written first, then the anion.
- In the formula of the complex ion/entity, the central metal atom is written first, then the ligands in alphabetical order.
- The formula of the entire coordination entity is enclosed in square brackets.
Rules for Naming:
Rule 1: Names of neutral coordination complexes are given without spaces. Cation is named first, separated by a space from the anion.
Rule 2 (Naming ligands first):
- Ligands that act as anions end in –o: Cl⁻ = chlorido, Br⁻ = bromido, I⁻ = iodido
- Anions ending in –ite and –ate are replaced with –ito and –ato: SO₄²⁻ = sulphato, CO₃²⁻ = carbonato, NO₂⁻ = nitrito, CH₃COO⁻ = acetato
- Neutral ligands get the same name as the uncoordinated molecule (with spaces omitted): C₅H₅N = pyridine, (CH₃)₂SO = dimethylsulfoxide (DMSO)
Exceptions — neutral ligands with special names:
| Molecule |
Special Name |
| H₂O |
aqua |
| NH₃ |
ammine |
| CS |
thiocarbonyl |
| CO |
carbonyl |
| NO |
nitrosyl |
Rule 3 (Prefixes): Greek prefixes (di, tri, tetra) are used for simple ligand names. For polydentate ligands (i.e., those with a binding site name containing di/tri already): bis-, tris-, tetrakis-, pentakis-, hexakis- are used instead. e.g., bis(ethane-1,2-diamine) not "diethylenediamine".
Rule 4: Oxidation state of the metal is indicated by a Roman numeral in parentheses after the metal name. NO = nitrosyl.
Rule 5 (Complex ion is a cation): Metal is named same as the element. e.g., Co in a cationic complex = cobalt. Name = Ligands + Metal name (with OS)
Rule 6 (Complex ion is an anion): Metal name ends in –ate + oxidation number.
Anionic Complex Metal Names:
| Metal |
Name in Anionic Complex |
| Iron |
Ferrate |
| Lead |
Plumbate |
| Gold |
Aurate |
| Chromium (Cr) |
Chromate |
| Palladium (Pd) |
Palladinate |
| Mercury (Hg) |
Mercurate |
| Zinc (Zn) |
Zincate |
| Nickel (Ni) |
Nickelate |
| Copper |
Cuprate |
| Silver |
Argentate |
| Tin |
Stannate |
| Cobalt (Co) |
Cobaltate |
| Platinum (Pt) |
Platinate |
| Cadmium (Cd) |
Cadmate |
| Aluminium (Al) |
Aluminate |
IUPAC Name Examples
- Na₂[Fe(CN)₅NO]: Sodium pentacyanonitrosatoferrate(II) (Note: pentacyanonitrosylferrate(II))
- [Fe(CN)₆]³⁻: hexacyanidoferrate(III) ion
- [Pt(NH₃)₂(Br)(NO₂)Cl]Cl: triamminebromochloronitroplatinum(IV) chloride
- K₃[Cr(C₂O₄)₃]: potassium trioxalatochromate(III)
Order of naming ions: Positive ion (cation) first, then negative ion (anion). In naming the complex ion, ligands first (alphabetically), then metal.
Key Points: Isomerism in Coordination Compounds
| Main Type |
Subtype |
Condition / Description |
Key Rule |
Example |
| Stereoisomerism |
Geometrical (cis–trans) |
Different spatial arrangement |
cis = 90°, trans = 180° |
[Pt(NH₃)₂Cl₂] |
| |
Optical |
Non-superimposable mirror images |
No plane of symmetry |
[Co(en)₃]³⁺ |
| Structural Isomerism |
Ionisation |
Exchange of ions inside/outside coordination sphere |
Counter ion acts as ligand |
[Co(NH₃)₅SO₄]Br / [Co(NH₃)₅Br]SO₄ |
| |
Linkage |
Ambidentate ligand attaches via different atoms |
NO₂⁻, SCN⁻ |
[Co(NH₃)₅NO₂]Cl₂ / [Co(NH₃)₅ONO]Cl₂ |
| |
Coordination |
Ligand exchange between metal complexes |
Two metal centers involved |
[Co(NH₃)₆][Cr(CN)₆] |
| |
Solvate (Hydrate) |
Solvent inside vs outside coordination sphere |
Crystal water difference |
[Cr(H₂O)₆]Cl₃ / [Cr(H₂O)₅Cl]Cl₂·H₂O |
Key Points: Applications of Coordination Compounds
| No. |
Application |
Description / Key Point |
Example |
| 1 |
Qualitative Analysis |
Used for the detection of metal ions |
Ni²⁺ + DMG → Ni-DMG (red ppt) |
| 2 |
Gravimetric Analysis |
Metal ions are estimated by forming stable complexes |
Conversion into stable coordination compounds |
| 3 |
Volumetric Analysis |
EDTA is used as a chelating agent in titrations |
Estimation of Ca²⁺, Mg²⁺, Zn²⁺ |
| 4 |
Biological Systems |
Essential role in living organisms |
Haemoglobin (Fe²⁺), Chlorophyll (Mg), Vitamin B₁₂ (Co) |
| 5 |
Medicinal Uses |
Used in the treatment of diseases |
Cis-platin [Pt(NH₃)₂Cl₂] (anti-cancer) |
| 6 |
Qualitative Separation |
Separation based on the stability of complexes |
Cu²⁺ & Cd²⁺ via cyanide complexes |
| 7 |
Photography |
Dissolution of AgBr using complex formation |
[Ag(S₂O₃)₂]³⁻ (hypo solution) |
| 8 |
Hydrometallurgy |
Extraction of metals using complexes |
[Ag(CN)₂]⁻, [Au(CN)₂]⁻ |
| 9 |
Electroplating |
Provides smooth and uniform coating |
Metal deposition using complexes |
Key Points: Crystal Field Theory (CFT)
- CFT assumes that ligands are point charges or dipoles and metal–ligand interaction is purely electrostatic.
- In an isolated metal ion, all five d-orbitals are degenerate (equal in energy).
- In an octahedral field, d-orbitals split into two sets:
Lower energy: t2g(dxy, dyz, dxz)
Higher energy: eg (dx²–y², dz²)
- The energy difference between t₂g and e_g orbitals is called crystal field splitting energy (Δ₀), and Δ₀ = 10 Dq.
- Energy change in octahedral field:
t2g orbitals are stabilized by −2/5 Δ₀
eg orbitals are destabilized by +3/5 Δ₀
- Magnitude of Δ₀ depends on ligand strength and oxidation state of the metal ion.
Strong field ligands → large Δ₀ → low spin
Weak field ligands → small Δ₀ → high spin
- Colour of coordination compounds is due to d–d transitions, where electrons absorb energy equal to Δ₀ and move from t2g to eg orbitals.
Key Points: Valence bond theory (VBT)
Valence Bond Theory (VBT) explains bonding in coordination compounds using hybridisation of metal orbitals.
- The central metal ion provides vacant orbitals (s, p and d) which hybridize to form equivalent hybrid orbitals.
- The number of hybrid orbitals formed equals the coordination number of the metal ion.
- Coordinate bonds are formed by overlap of vacant hybrid orbitals of the metal ion with filled orbitals of ligands.
- Hybridisation determines geometry:
- sp³ → Tetrahedral
- dsp² → Square planar
- d²sp³ → Octahedral (inner orbital)
- sp³d² → Octahedral (outer orbital)
- Inner orbital complexes (d²sp³) involve pairing of (n−1)d electrons before hybridisation and are generally low spin.
- Outer orbital complexes (sp³d²) do not involve pairing of (n−1)d electrons before hybridisation and are generally high spin; magnetic nature depends on the number of unpaired electrons.
Key Points: Werner’s Theory of Coordination Complexes
Werner’s Theory of Coordination Complexes explains the bonding and structure of coordination compounds through the concept of valencies of the metal ion.
- The central metal ion exhibits two types of valencies, namely primary valency and secondary valency.
- Primary valency is ionizable and corresponds to the oxidation state of the metal ion, and it is generally satisfied by anions present outside the coordination sphere.
- Secondary valency is non-ionizable and corresponds to the coordination number of the metal ion, and it is satisfied by ligands directly bonded to the metal ion.
- Secondary valencies are fixed in number and have definite spatial arrangement, which determines the geometry of the complex, such as octahedral, tetrahedral, or square planar.
