Coordination Compounds Notes
Study Notes
Topics
8Chapter Overview
Overview
Coordination compounds are substances in which a central metal atom or ion is surrounded by ions or molecules called ligands through coordinate bonds. This chapter connects structure, naming, isomerism, bonding, magnetism, colour and applications. Werner’s theory explains primary and secondary valencies, while IUPAC nomenclature gives a systematic way to name complexes. Isomerism explains why compounds with the same formula may show different structures or properties. Valence Bond Theory and Crystal Field Theory explain geometry, hybridisation, magnetic behaviour and colour. For NEET, the highest-yield areas are oxidation number, coordination number, IUPAC naming, types of isomerism, crystal field splitting, high-spin/low-spin complexes and biological or analytical applications.
- 1Coordination compounds often retain their identity in solution because the coordination sphere is stable.
- 2Ligands may be neutral, anionic or cationic and may donate through one or more donor atoms.
- 3The same metal can have different coordination numbers and geometries in different complexes.
- 4Strong field ligands cause larger crystal field splitting and often produce low-spin complexes.
- 5Colour arises mainly due to d-d transitions or charge transfer transitions.
- 6Applications include haemoglobin, chlorophyll, vitamin B12, EDTA titration, electroplating and cancer therapy.
Chapter Order Mnemonic
Remember the flow as W-N-I-B-A: Werner, Nomenclature, Isomerism, Bonding, Applications.
Core Question Trick
For any complex ask: Who is metal? What are ligands? What is charge? What is CN? What is geometry?
Daily-Life Connection
Haemoglobin is a coordination compound of iron where ligands around Fe help transport oxygen in blood.
Analytical Connection
EDTA forms stable complexes with Ca2+ and Mg2+ and is used to estimate hardness of water.
Confusing Coordination Number with Oxidation Number
Coordination number counts donor atoms attached to metal, while oxidation number is a formal charge calculated using ligand charges.
Ignoring Square Brackets
Species inside brackets remain coordinated; species outside brackets are ionisable counter ions.
Used to calculate the oxidation state of the central metal ion in a coordination entity.
Variables
x=Oxidation number of central metal
sum(charges of ligands)=Total charge contributed by all ligands
charge on complex ion=Net charge written outside the square bracket
Werner Theory & Basic Concepts
Overview
Werner’s theory was the first successful explanation of coordination compounds. It states that metals show two types of valencies: primary valency, which is ionisable and satisfied by anions, and secondary valency, which is non-ionisable and satisfied by ligands. The central metal atom or ion with attached ligands is called a coordination entity. Ligands donate electron pairs through donor atoms and may be monodentate, bidentate or polydentate. The coordination number is the number of donor atoms directly attached to the metal, not the number of ligands. The coordination sphere is the part inside square brackets, while ions outside are counter ions. These basics are essential for naming, isomerism and bonding.
- 1Werner explained compounds like CoCl3·6NH3 using ionisable and non-ionisable chloride ions.
- 2Neutral ligands like NH3, H2O and CO contribute zero charge while calculating oxidation number.
- 3Anionic ligands like Cl−, CN−, OH− and NO2− affect the oxidation number calculation.
- 4Chelating ligands form ring structures and generally increase complex stability.
- 5Coordination number commonly 2, 4 and 6 gives linear, tetrahedral/square planar and octahedral geometries respectively.
- 6The coordination sphere behaves as a single unit in many reactions.
Primary vs Secondary Valency
Primary = ionisable = outside bracket possible; Secondary = fixed direction = inside bracket bonding.
CN Counting Trick
Count hands touching the metal, not the number of people. A bidentate ligand has two hands.
Oxidation Number Example
In [Co(NH3)5Cl]Cl2, the complex ion has charge +2. NH3 is neutral and coordinated Cl is -1, so x - 1 = +2; Co is +3.
Coordination Number Example
In [Fe(CN)6]4−, six cyanido ligands are directly attached to Fe, so CN = 6.
Counting Ligands Instead of Donor Atoms
[Co(en)3]3+ has three ligands but CN is 6 because each en donates through two nitrogen atoms.
Taking Neutral Ligands as Charged
NH3, H2O, CO and NO as nitrosyl in many school-level examples are commonly treated carefully; NH3, H2O and CO are neutral.
For monodentate ligands, CN often equals number of ligands; for bidentate or polydentate ligands, multiply by denticity.
Variables
CN=Coordination number
donor atoms=Atoms of ligands directly bonded to the central metal
Nomenclature
Overview
Nomenclature of coordination compounds follows fixed IUPAC rules. In the name, cation is named before anion, whether the complex is cationic, anionic or neutral. Within the coordination sphere, ligands are named before the metal, and ligands are arranged alphabetically, ignoring prefixes such as di, tri, tetra. Anionic ligands usually end in -ido, while neutral ligands have special names such as ammine, aqua and carbonyl. The oxidation state of the metal is written in Roman numerals in parentheses. If the complex ion is anionic, the metal name often ends with -ate, such as ferrate, cuprate, argentate or cobaltate. Correct naming is a very frequent NEET testing area.
- 1The number of each ligand is shown by multiplicative prefixes.
- 2Alphabetical order is based on ligand name, not prefix.
- 3Counter ions are named separately from the coordination entity.
- 4Anionic ligand names include chlorido, bromido, cyanido, hydroxido and nitrito depending on donor atom.
- 5If ligand itself contains a prefix or is polydentate, use bis, tris or tetrakis.
- 6Oxidation state must be calculated before final naming.
Naming Order Trick
CLM: Cation, Ligand, Metal. First decide charge, then alphabetically arrange ligands, then write metal with Roman numeral.
Neutral Ligands
Aqua, Ammine, Carbonyl: AAC are common neutral ligand names you must spell correctly.
Example 1
[Co(NH3)6]Cl3 is named hexaamminecobalt(III) chloride. Co is +3 because three chloride ions are outside.
Example 2
K4[Fe(CN)6] is potassium hexacyanidoferrate(II). The complex is anionic, so iron becomes ferrate.
Example 3
[Ni(CO)4] is tetracarbonylnickel(0). CO is neutral, so Ni has oxidation state zero.
Using Ammonia Instead of Ammine
NH3 as a ligand is called ammine, not ammonia.
Alphabetising Prefixes
Do not alphabetise using di, tri or tetra. Alphabetise by ligand name.
Forgetting -ate in Anionic Complex
[Fe(CN)6]4− is hexacyanidoferrate(II), not hexacyanidoiron(II).
General order for writing the IUPAC name of coordination compounds.
Variables
counter ion=Ion outside the coordination sphere, named according to cation-anion order
ligands=Groups directly attached to the metal
metal(oxidation state)=Central metal followed by Roman numeral
Isomerism
Overview
Isomerism in coordination compounds occurs when compounds have the same molecular formula but different arrangements of atoms or ligands. Structural isomerism includes linkage isomerism, coordination isomerism, ionisation isomerism and hydrate isomerism. In linkage isomerism, an ambidentate ligand attaches through different donor atoms, such as NO2− through N or O. Coordination isomerism occurs when ligands are exchanged between cationic and anionic complex ions. Stereoisomerism includes geometrical and optical isomerism. Geometrical isomerism is common in square planar and octahedral complexes, giving cis-trans or fac-mer forms. Optical isomerism arises when non-superimposable mirror images exist. NEET often asks direct identification of isomerism from formulas and structures.
- 1Tetrahedral complexes usually do not show geometrical isomerism because all positions are equivalent.
- 2Square planar complexes show strong geometrical isomerism due to fixed 90° and 180° positions.
- 3Cis isomers have similar ligands adjacent; trans isomers have them opposite.
- 4Facial isomer has three identical ligands on one face of an octahedron; meridional has them around a meridian.
- 5Optical activity is common in chelated octahedral complexes like [Co(en)3]3+.
- 6Hydrate isomerism changes the number of water molecules inside and outside the coordination sphere.
Linkage Ligands
Remember NCS: NO2−, CN−, SCN− can create linkage confusion by changing donor atom.
fac vs mer
fac = face; three same ligands occupy one triangular face. mer = meridian; ligands lie around a line through the metal.
Linkage Example
[Co(NH3)5NO2]Cl2 and [Co(NH3)5ONO]Cl2 differ because NO2− attaches through N in nitro and through O in nitrito.
Geometrical Example
cisplatin, cis-[Pt(NH3)2Cl2], is an anticancer drug, while the trans isomer has different biological activity.
Assuming Tetrahedral MA2B2 Shows cis-trans
Tetrahedral positions are equivalent, so ordinary tetrahedral MA2B2 does not show cis-trans isomerism.
Confusing Ionisation and Coordination Isomerism
Ionisation isomerism exchanges an ion inside and outside the sphere; coordination isomerism exchanges ligands between two complex ions.
Applies to both structural and stereoisomers in coordination compounds.
Variables
same molecular formula=Same total atoms and ions
different arrangement=Different connectivity or spatial orientation
Bonding & Crystal Field Theory
Overview
Bonding in coordination compounds is explained by Valence Bond Theory and Crystal Field Theory. VBT describes hybridisation and geometry using vacant orbitals of the metal ion and lone-pair donation by ligands. It explains inner-orbital and outer-orbital complexes but does not satisfactorily explain colour and detailed magnetic behaviour. Crystal Field Theory treats ligands as point charges or dipoles that split the degenerate d-orbitals of the metal ion. In octahedral complexes, dxy, dyz and dxz orbitals form lower t2g set, while dx2-y2 and dz2 form higher eg set. The magnitude of splitting decides high-spin or low-spin arrangement, number of unpaired electrons, magnetic moment and colour due to electronic transitions.
- 1Octahedral complexes may use d2sp3 inner-orbital or sp3d2 outer-orbital hybridisation in VBT.
- 2Square planar complexes often occur for d8 metal ions like Ni2+, Pd2+ and Pt2+ with strong field ligands.
- 3Crystal field splitting energy is represented as Δo for octahedral and Δt for tetrahedral complexes.
- 4For the same metal and ligand, Δt is smaller than Δo, approximately Δt = 4/9 Δo.
- 5Pairing occurs when Δo is greater than pairing energy.
- 6A complex with no unpaired electrons is diamagnetic; with unpaired electrons it is paramagnetic.
- 7Spectrochemical series orders ligands by increasing field strength.
Octahedral Levels
In octahedral field, t2g is the basement and eg is the terrace: t2g lower, eg higher.
Strong Field
Strong field ligands force electrons to pair: CN− and CO are strong closers of spin.
Magnetic Moment Example
If a complex has 3 unpaired electrons, μ = √[3(3+2)] = √15 = 3.87 BM approximately.
High Spin Example
[FeF6]3− has Fe3+ as d5 and F− is weak field, so it is high spin with many unpaired electrons.
Low Spin Example
[Fe(CN)6]3− has Fe3+ as d5 and CN− is strong field, so pairing occurs and it has fewer unpaired electrons.
Applying High/Low Spin to All Tetrahedral Cases
Tetrahedral splitting is small, so tetrahedral complexes are generally high spin.
Forgetting Metal Oxidation State Before d Count
Always find oxidation state first, then d-electron configuration. Fe2+ is d6, Fe3+ is d5.
Confusing Diamagnetic with Colourless
Diamagnetic means no unpaired electrons; colour depends on electronic transitions and is not the same property.
Used to calculate approximate magnetic moment from unpaired electrons.
Variables
μ=Magnetic moment
n=Number of unpaired electrons
BM=Bohr Magneton
Tetrahedral splitting is smaller than octahedral splitting for comparable metal-ligand conditions.
Variables
Δt=Crystal field splitting energy in tetrahedral complex
Δo=Crystal field splitting energy in octahedral complex
Applications of Coordination Compounds
Overview
Coordination compounds are not just theoretical; they are central to biology, medicine, analysis, industry and metallurgy. In biological systems, haemoglobin contains iron and transports oxygen, chlorophyll contains magnesium and helps photosynthesis, and vitamin B12 contains cobalt. In analytical chemistry, EDTA forms stable chelates with metal ions and is used in complexometric titrations, especially hardness of water. In medicine, cisplatin is an important anticancer drug. In industry, coordination compounds are used in electroplating, photography and catalysis. In metallurgy, complex formation helps in extraction and purification of metals, such as cyanide complexes in gold and silver extraction and Mond’s process for nickel purification.
- 1Chelation increases stability of metal-ligand complexes.
- 2Biological metal complexes perform highly specific functions.
- 3Analytical applications depend on selective and stable complex formation.
- 4Industrial catalysts often use transition metal coordination compounds.
- 5Medicinal action of cisplatin depends on ligand substitution and binding to DNA.
- 6Metallurgical processes use complex formation to dissolve, separate or purify metals.
Bio Metals
He-Fe, Chlo-Mg, B12-Co: Haemoglobin has Fe, chlorophyll has Mg, vitamin B12 has Co.
Applications Mnemonic
BAMIM: Biological, Analytical, Medicinal, Industrial, Metallurgical applications.
Hard Water
Hard water contains Ca2+ and Mg2+. EDTA binds these ions strongly, allowing their quantitative estimation.
Gold Extraction
Gold dissolves in cyanide solution by forming a soluble complex, which helps separate it from ore.
Nickel Purification
Impure nickel forms volatile Ni(CO)4, which decomposes on heating to give pure nickel.
Forgetting Vitamin B12 Metal
Vitamin B12 contains cobalt, not iron or magnesium.
Mixing EDTA with Indicator
EDTA is the complexing agent in hardness estimation; indicators only show endpoint.
Confusing cisplatin with trans-platin
The medically important anticancer drug is cis-[Pt(NH3)2Cl2].
General complex formation used to estimate Ca2+ and Mg2+ ions in hard water.
Variables
M2+=Divalent metal ion such as Ca2+ or Mg2+
EDTA4−=Hexadentate chelating ligand
[M(EDTA)]2−=Stable metal-EDTA complex
Formula Sheet
Overview
This formula sheet collects the calculation tools needed for NEET questions on coordination compounds. The most common calculation is oxidation number of the metal, found by balancing ligand charges with the overall charge of the complex. Coordination number is obtained by counting donor atoms directly attached to the metal. Magnetic moment is calculated using the spin-only formula after finding the number of unpaired electrons from the d-electron configuration and ligand field strength. Crystal field splitting relations help compare octahedral and tetrahedral complexes, while CFSE expressions show stabilisation due to electron placement in split d-orbitals. Use these formulas only after correctly identifying ligand charge, denticity, geometry and metal oxidation state.
- 1Formula questions are usually simple if ligand charge and denticity are known.
- 2Neutral ligands do not affect oxidation number but do affect coordination number.
- 3Bidentate ligands double the donor count per ligand molecule.
- 4For transition metals, d-electron count equals group number minus oxidation state in many NEET-level examples.
- 5Magnetic moment is zero for diamagnetic complexes with no unpaired electrons.
- 6Always check whether the complex is octahedral, tetrahedral or square planar before predicting spin.
Calculation Order
Charge → Oxidation state → d count → field strength → unpaired electrons → magnetic moment.
CN Example
[Co(en)2Cl2]+ has two en ligands contributing 4 donor atoms and two Cl− contributing 2, so CN = 6.
Magnetic Formula Example
For n = 2, μ = √[2(4)] = √8 = 2.83 BM.
Using Atomic Number Directly as d Count
Remove electrons according to oxidation state first. Co3+ is not the same d count as neutral Co.
Forgetting Denticity
Three en ligands give CN 6, not CN 3.
Finds the oxidation state of the central metal ion.
Variables
x=Oxidation number of metal
Σ(ligand charges)=Sum of all ligand charges
charge on complex=Net charge of coordination entity
Gives the total number of donor atoms bonded to the central metal.
Variables
CN=Coordination number
denticity=Number of donor atoms per ligand
Calculates approximate magnetic moment for paramagnetic complexes.
Variables
μ=Magnetic moment
n=Number of unpaired electrons
Quick Revision
Overview
This quick revision section compresses the whole chapter into a NEET-ready checklist. Start with the coordination entity: identify the metal, ligands, ligand charges, coordination number and oxidation number. For naming, remember the order cation first, then ligands alphabetically, then metal with Roman numeral, and use -ate for anionic complexes. For isomerism, first decide whether the difference is in bonding or spatial arrangement. For bonding, calculate the metal d count and use ligand strength to decide high spin or low spin. Finally, connect applications to NCERT examples such as haemoglobin, chlorophyll, vitamin B12, EDTA, cisplatin, cyanide extraction and Mond’s process.
- 1Always solve coordination questions in a fixed sequence rather than by guesswork.
- 2Naming and oxidation number questions are highly scoring if rules are memorised.
- 3Isomerism questions require checking formula, ligand type, geometry and symmetry.
- 4CFT questions depend on d-electron count, ligand strength and geometry.
- 5Applications are NCERT fact-based and should be revised as a table.
Five-Step NEET Drill
M-L-C-O-G: Metal, Ligands, Charge, Oxidation state, Geometry.
Rapid Question Approach
For [Co(NH3)4Cl2]+: NH3 neutral, two Cl− = -2, complex charge +1, so Co = +3; CN = 6; octahedral; possible cis-trans.
Skipping Charge Balance
Most wrong answers in naming and magnetism start from a wrong oxidation number.
Not Identifying Geometry Before Isomerism
Geometrical isomerism depends strongly on geometry; square planar and tetrahedral MA2B2 behave differently.
The fastest formula for oxidation state questions.
Variables
complex charge=Charge on coordination sphere
metal ON=Oxidation number of central metal
total ligand charge=Sum of ligand charges
Mind Map
Overview
The mind map of coordination compounds begins with a central metal ion surrounded by ligands. From this core, the chapter branches into basic terminology, nomenclature, isomerism, bonding and applications. Basic terminology includes coordination entity, coordination sphere, ligand, denticity, coordination number and oxidation number. Nomenclature converts formulas into systematic names using ligand order, metal name and oxidation state. Isomerism explains different structures from the same formula. Bonding connects ligand field, hybridisation, crystal field splitting, spin, magnetism and colour. Applications connect the chapter to living systems, analysis, medicines, industry and metallurgy. Use this map to revise the chapter in one visual pass before attempting NEET questions.
- 1A coordination compound question usually belongs to one of five branches: basics, naming, isomerism, bonding or applications.
- 2Oxidation number and coordination number are central connectors in the mind map.
- 3Ligand type affects naming, isomerism, splitting and stability.
- 4Geometry is a bridge concept between Werner theory, VBT, CFT and isomerism.
- 5Applications are easiest when memorised as compound-metal-function.
Mind Map Mnemonic
B-N-I-B-A: Basics, Naming, Isomerism, Bonding, Applications.
Mind Map Use Example
For [Pt(NH3)2Cl2], the mind map asks: basics give Pt oxidation state and CN, naming gives diamminedichloridoplatinum(II), isomerism gives cis-trans, and application recalls cisplatin.
Studying Topics Separately Without Links
Nomenclature, isomerism and CFT all require oxidation number, ligand identity and geometry. Revise them together.
The conceptual formula behind the entire chapter.
Variables
Metal=Central atom or ion accepting electron pairs
Ligands=Electron-pair donors attached to metal
Coordination Entity=Complex species written inside square brackets
Formula Sheet
10Used to calculate the oxidation state of the central metal ion in a coordination entity.
Variables
x=Oxidation number of central metal
sum(charges of ligands)=Total charge contributed by all ligands
charge on complex ion=Net charge written outside the square bracket
Approximate magnetic moment of a complex due to unpaired electrons.
Variables
μ=Magnetic moment in Bohr Magneton
n=Number of unpaired electrons
BM=Bohr Magneton unit
For monodentate ligands, CN often equals number of ligands; for bidentate or polydentate ligands, multiply by denticity.
Variables
CN=Coordination number
donor atoms=Atoms of ligands directly bonded to the central metal
Used to determine the oxidation number of the central metal.
Variables
ON(M)=Oxidation number of metal
ligand charge total=Sum of charges on all ligands
complex charge=Overall charge on coordination entity
General order for writing the IUPAC name of coordination compounds.
Variables
counter ion=Ion outside the coordination sphere, named according to cation-anion order
ligands=Groups directly attached to the metal
metal(oxidation state)=Central metal followed by Roman numeral
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NEET PYQs — Coordination Compounds
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Match List I with List II: Choose the correct answer from the options given below :
Which one of the following is an ambidentate ligand?
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