Subtopics - Coordination Chemistry (NEET)
Comprehensive coverage of coordination compound theory, bonding, isomerism, and applications for NEET
1) Double Salts and Coordination Compounds
Double salts lose their identity in solution and give all constituent ions. Coordination compounds retain their identity even in solution. K2[Ni(CN)4] gives only 2K+ and [Ni(CN)4]2 minus in solution, not free Ni2+ or CN minus. Molar conductivity of these solutions corresponds only to ions actually released, which is a key experimental distinction between double salts and coordination compounds.
2) Terminology of Coordination Compounds
Central metal ion acts as a Lewis acid with empty d-orbitals to accept electron pairs from ligands. Ligands are Lewis bases classified as unidentate (F minus, Cl minus, NH3, H2O), bidentate (ethylenediamine, oxalate), and hexadentate (EDTA with 6 donor atoms). Coordination number is the total number of ligand donor atoms bonded to the central ion. Coordination sphere is written inside square brackets. Oxidation number is determined by the algebraic sum of charges on central ion and ligands.
3) IUPAC Nomenclature of Coordination Compounds
Cation is named first, anion second. Ligands are named before the metal in the coordination sphere. Neutral ligands use molecule name with exceptions: H2O is aquo, NH3 is ammine, CO is carbonyl, NO is nitrosyl. Negative ligands end in -o (chloro, cyano, oxalato, hydroxo). Ligand prefixes use di, tri, tetra for simple ligands and bis, tris, tetrakis for complex ligands. When the complex is anionic, the metal name ends in -ate (ferrate, cobaltate, platinate). Oxidation state is indicated by Roman numerals in parentheses. The entire coordination sphere is written as one word with no spaces.
4) Preparation of Coordination Compounds
Three main methods: ligand substitution (replacing H2O ligands with stronger ligands like NH3 or CN minus), direct mixing of reagents (PtCl2 with ethylenediamine), and redox reactions (cobalt(II) oxidised to cobalt(III) by H2O2 during complex formation). Ligand substitution is the most common route and exploits the relative field strengths of incoming and leaving ligands.
5) Isomerism in Coordination Compounds
Structural isomerism includes ionisation isomerism (different ions outside the coordination sphere), hydrate isomerism (different number of H2O inside vs outside), coordination isomerism (interchange of ligands between cationic and anionic parts), linkage isomerism (ambidentate ligands like NO2 minus bonding through N or O, SCN minus through S or N), polymerisation isomerism, and coordination position isomerism. Stereoisomerism includes geometrical (cis-trans) isomerism in square planar and octahedral complexes, and optical isomerism in complexes lacking a plane of symmetry. Key rule: tetrahedral complexes do not show geometrical isomerism. Ma6 and Ma5b octahedral complexes do not show geometrical isomerism either.
6) VBT for Bonding in Coordination Compounds
Valence Bond Theory by Pauling explains geometry through hybridisation: sp for linear (CN 2), sp3 for tetrahedral (CN 4), dsp2 for square planar (CN 4), d2sp3 for inner orbital octahedral (CN 6), and sp3d2 for outer orbital octahedral (CN 6). Strong ligands (CN minus, CO, en, NH3) force electron pairing, favouring inner orbital (low-spin) complexes. Weak ligands (F minus, Cl minus, Br minus) cannot force pairing, yielding outer orbital (high-spin) complexes. Magnetic moments distinguish between inner and outer orbital complexes. VBT limitations: cannot explain colour, cannot quantitatively distinguish ligand strengths, cannot predict kinetic stability.
7) Crystal Field Theory
CFT treats metal-ligand interaction as electrostatic. In octahedral field, five d-orbitals split into lower-energy t2g (dxy, dyz, dxz) and higher-energy eg (dx2-y2, dz2) sets separated by energy Delta-oct. In tetrahedral field, splitting is inverted and smaller: Delta-t equals 4/9 of Delta-oct. When Delta-oct is large (strong-field ligands), electrons preferentially pair in t2g before filling eg, forming low-spin complexes. When Delta-oct is small (weak-field ligands), electrons fill all five d-orbitals singly first (Hund rule), forming high-spin complexes. Low-spin configurations only exist for d4 through d7 in octahedral complexes. The spectrochemical series orders ligands by increasing field strength: I minus < Br minus < Cl minus < F minus < OH minus < H2O < C2O4 2 minus < NH3 < en < NO2 minus < CN minus. Colour arises from d-d transitions absorbing visible light; the complementary colour is observed.
8) Factors Affecting the Stability of Complex Ion
Stability depends on charge density of the central metal ion (higher charge, smaller size gives greater stability), base strength of the ligand (CN minus forms more stable complexes than NH3), and chelate effect (polydentate ligands like ethylenediamine form far more stable complexes than equivalent monodentate ligands). [Fe(CN)6]3 minus with K = 1.2 times 10 to the 31 is far more stable than [Fe(CN)6]4 minus with K = 1.8 times 10 to the 6. Chelating agent EDTA exploits the chelate effect for water purification and treatment of metal poisoning.
9) Shape of Complexes
Octahedral complexes form via d2sp3 (inner orbital, low spin) or sp3d2 (outer orbital, high spin) hybridisation. [Cr(NH3)6]3+ is inner octahedral with 3 unpaired electrons (paramagnetic). [Fe(CN)6]4 minus is inner octahedral, diamagnetic after CN minus forces pairing. [CoF6]3 minus is outer octahedral with 4 unpaired electrons. Tetrahedral complexes form via sp3 hybridisation; [Zn(NH3)4]2+ and [Ni(CO)4] are tetrahedral and diamagnetic. Square planar complexes form via dsp2 hybridisation; [Ni(CN)4]2 minus is square planar and diamagnetic. [Cu(NH3)4]2+ is confirmed square planar by X-ray analysis despite theoretical ambiguity between sp3 and dsp2.
10) Applications of Coordination Chemistry
Applications in qualitative analysis: AgCl dissolves in NH3 forming [Ag(NH3)2]+; Cu2+ is masked by excess CN minus as [Cu(CN)4]3 minus allowing selective precipitation of CdS. Electroplating uses cyano complexes for uniform metal deposition. EDTA chelation purifies hard water and treats lead poisoning (Pb2+ displaces Ca2+ from [Ca(EDTA)]2 minus because K-f of [Pb(EDTA)]2 minus is 10 to the 18). Extraction of gold uses cyanide complexation followed by Zn displacement. Mond process exploits Ni(CO)4 for nickel purification. Dimethylglyoxime gives cherry-red chelate with Ni2+ for detection. Cisplatin [Pt(NH3)2Cl2] is used in cancer treatment.
Subtopics - Coordination Chemistry (NEET)
Comprehensive coverage of coordination compound theory, bonding, isomerism, and applications for NEET
11) Organometallic Compounds
Organometallic compounds contain at least one metal-carbon bond. They are classified into sigma-bonded (RMgX, R2Zn, R4Sn), pi-bonded (ferrocene with eta-5 cyclopentadienyl, dibenzene chromium with eta-6 benzene, Zeise salt K[PtCl3(eta-2-C2H4)]), and compounds with both sigma and pi bonding (metal carbonyls: Ni(CO)4, Fe(CO)5, Cr(CO)6). Applications include Grignard reagent for organic synthesis, Wilkinson catalyst [(Ph3P)3RhCl] for alkene hydrogenation, and Ziegler-Natta catalyst (TiCl4 plus trialkylaluminium) for alkene polymerisation.
Coordination Chemistry Download Notes & Weightage Plan
For each topic in the Coordination Chemistry chapter below, you get (2) the exact resources to download and how to use them, and (3) a simple importance & time plan so NEET students know what to do first and what to revise last.
Double Salts and Coordination Compounds
Distinction between double salts and coordination compounds based on solution behaviour.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Conductivity-based identification of double salt versus coordination compound
- High-risk Area: Assuming all addition compounds are coordination compounds. Double salts like Mohr salt dissociate completely in solution.
- Best Practice Style: Concept mapping with ion counting
Terminology of Coordination Compounds
Central ion, ligands, denticity, coordination number, coordination sphere, oxidation number.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Coordination number determination and oxidation state calculation
- High-risk Area: Confusing coordination number with the number of ligands when polydentate ligands are present.
- Best Practice Style: Table-based drill with worked examples
IUPAC Nomenclature of Coordination Compounds
Naming rules for cationic, anionic, and neutral complexes.
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Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Correct application of -o suffix, alphabetical order, -ate ending for anionic complexes
- High-risk Area: Forgetting to use -ate for anionic complexes (ferrate, not iron). Confusing ammine (NH3 ligand) with amine (organic compound).
- Best Practice Style: Extensive naming drill with timed practice
Isomerism in Coordination Compounds
Six types of structural isomerism and two types of stereoisomerism in coordination compounds.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Identifying the correct type of isomerism from a given pair of formulas; knowing which complexes can show geometrical or optical isomerism
- High-risk Area: Claiming tetrahedral complexes show geometrical isomerism. Confusing hydrate isomerism with ionisation isomerism.
- Best Practice Style: Classification flowchart plus worked examples
VBT for Bonding in Coordination Compounds
Hybridisation-based prediction of geometry and magnetic properties.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Identifying hybridisation from ligand type and predicting paramagnetic vs diamagnetic
- High-risk Area: Not recognising that NH3 sometimes acts as a weak ligand and H2O sometimes as a strong ligand (borderline behaviour). Confusing d2sp3 with sp3d2.
- Best Practice Style: Orbital filling diagrams with colour-coded boxes
d-orbital splitting, spectrochemical series, high-spin vs low-spin, and colour of complexes.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Predicting spin state from ligand strength, calculating magnetic moment from unpaired electrons, relating absorbed colour to observed colour
- High-risk Area: Forgetting that low-spin configurations only exist for d4 through d7 in octahedral. Confusing spectral colour (absorbed) with complementary colour (observed).
- Best Practice Style: t2g/eg filling tables plus colour wheel
Applications of Coordination Chemistry
Qualitative analysis, electroplating, EDTA chelation, metallurgy, and medicinal applications.
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2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Identifying the coordination compound involved in each application
- High-risk Area: Not knowing that cisplatin is the cis isomer specifically (the trans isomer is inactive).
- Best Practice Style: Application-reaction-principle table
Classification, examples, and applications of organometallic compounds.
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2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Classification of organometallic compounds and identification of sigma vs pi bonding
- High-risk Area: Confusing coordination compounds with organometallic compounds. Not all metal complexes are organometallics; the M-C bond is the defining feature.
- Best Practice Style: Classification table with structural formulas
Coordination Chemistry Chapter NEET Traps & Common Mistakes (Topic-Wise)
Each subtopic below is of the Coordination Chemistry chapter and shows what NEET students usually do wrong in NEET examination, a short example of the mistake, and how NEET frames the question to trick you with close options are given below.
Mistake Snapshot (What Students Do Wrong)
- Treating each ligand molecule as contributing 1 to coordination number: When a polydentate ligand like EDTA (hexadentate) bonds to a metal, it contributes 6 to the coordination number, not 1. The coordination number counts donor atoms, not ligand molecules.
- Confusing coordination number with oxidation state: In [Co(NH3)6]3+, the coordination number is 6 (six donor atoms) but the oxidation state is +3. These are independent quantities determined by different rules.
What is the coordination number of Fe in [Fe(C2O4)3]3 minus? Each oxalate is bidentate, so CN = 3 times 2 = 6, not 3.
How NEET Frames The Trap
NEET asks the coordination number of a metal in a complex with polydentate ligands. Students who count ligands instead of donor atoms get the wrong answer.
Q. The coordination number of cobalt in [Co(EDTA)] minus is
A. 6 B. 1 C. 4 D. 8
Trick: 6 (Option A): EDTA is a hexadentate ligand with 6 donor atoms (2 N and 4 O), each forming a coordinate bond with Co. Option B counts only the single EDTA molecule. Option C counts only the 4 carboxylate groups. Option D incorrectly adds the 2 N plus 4 O plus 2 extra.
Mistake Snapshot (What Students Do Wrong)
- Using the normal metal name for anionic complexes: When the complex ion is an anion, the metal name must end in -ate. [Fe(CN)6]4 minus is hexacyanoferrate(II), not hexacyanoiron(II). Students who forget -ate lose marks on direct naming questions.
- Confusing ammine with amine: Ammine (double m) is the IUPAC name for NH3 as a ligand. Amine is an organic functional group. Using the wrong spelling in a naming question is penalised.
Name [Cr(NH3)2Cl4] minus. Answer: diamminetetrachlorochromate(III). Metal ends in -ate because the complex is an anion.
How NEET Frames The Trap
NEET often gives a complex formula and asks for the IUPAC name, with options that differ only in whether -ate is used. Students who miss the anionic nature choose the wrong option.
Q. The correct IUPAC name of K3[Fe(CN)6] is
A. potassium hexacyanoferrate(III) B. potassium hexacyanoiron(III) C. potassium hexacyanoferrate(II) D. tripotassium hexacyanoiron(III)
Trick: Potassium hexacyanoferrate(III) (Option A): The complex ion [Fe(CN)6]3 minus is anionic, so the metal name must end in -ate (ferrate, not iron). Option B omits -ate. Option C gives the wrong oxidation state (+2 instead of +3). Option D uses tripotassium, which is not standard IUPAC.
Mistake Snapshot (What Students Do Wrong)
- Claiming tetrahedral complexes show geometrical isomerism: In a regular tetrahedron, all four positions are equivalent and adjacent to each other. There is no distinction between cis and trans positions. Therefore, tetrahedral complexes of any formula type never exhibit geometrical isomerism.
- Forgetting conditions for octahedral GI: Not all octahedral complexes show GI. Ma6 and Ma5b types do not show GI because all positions of the single different ligand are equivalent. GI requires at least two different ligands with the formula Ma4b2 or Ma3b3 and above.
[NiCl2(PPh3)2] is tetrahedral. A student claims it has cis and trans isomers. This is incorrect because tetrahedral geometry has all-equivalent positions.
How NEET Frames The Trap
NEET asks which of the following complexes can show geometrical isomerism. A tetrahedral complex is included as a distractor. Students who do not recall that tetrahedral geometry prevents GI select it.
Q. Which of the following shows geometrical isomerism?
A. [Pt(NH3)2Cl2] (square planar) B. [ZnCl2Br2]2 minus (tetrahedral) C. [Ni(CO)4] (tetrahedral) D. [Ag(NH3)2]+ (linear)
Trick: [Pt(NH3)2Cl2] (Option A): Square planar Ma2b2 complexes show cis-trans isomerism because two distinct spatial arrangements exist. Options B and C are tetrahedral, which never shows GI. Option D is linear with only one arrangement possible.
Mistake Snapshot (What Students Do Wrong)
- Trying to classify d1 to d3 or d8 to d10 as low spin: For d1, d2, d3, d8, d9, and d10 configurations in octahedral field, the electron arrangement is the same regardless of ligand field strength. The distinction between high-spin and low-spin only applies to d4 through d7.
- Using the wrong spectrochemical series order: Students sometimes swap the positions of H2O and NH3. The correct order places H2O before NH3: I minus < Br minus < Cl minus < F minus < OH minus < H2O < oxalate < NH3 < en < NO2 minus < CN minus.
[Ti(H2O)6]3+ has d1 configuration. A student labels it low-spin because H2O is between weak and strong. This classification is meaningless for d1 because only one electron is present regardless of splitting.
How NEET Frames The Trap
NEET pairs a d3 metal with CN minus and asks whether the complex is high-spin or low-spin. The correct answer is that the distinction does not apply to d3; the filling is always the same.
Q. Which of these d-electron configurations forms both high-spin and low-spin octahedral complexes?
A. d5 B. d2 C. d8 D. d10
Trick: d5 (Option A): For d4 through d7, the electron filling pattern differs depending on whether Delta-oct exceeds the pairing energy. d2 (Option B), d8 (Option C), and d10 (Option D) have identical high and low-spin arrangements, so the distinction does not apply.
Mistake Snapshot (What Students Do Wrong)
- Assuming both cis and trans isomers of [Pt(NH3)2Cl2] are anticancer agents: Only the cis isomer (cisplatin) shows anticancer activity by cross-linking DNA strands. The trans isomer (transplatin) does not show this activity because the geometry prevents effective cross-linking.
- Not recognising cisplatin as a square planar complex: Students sometimes assume [Pt(NH3)2Cl2] is tetrahedral. Pt(II) with d8 configuration and strong-field ligands forms square planar complexes. Tetrahedral Pt(II) would not show cis-trans isomerism.
Why is only cis-[Pt(NH3)2Cl2] used as an anticancer drug? Because the two Cl atoms in cis position can bridge adjacent DNA bases, disrupting replication. The trans isomer cannot achieve this bridging geometry.
How NEET Frames The Trap
NEET asks about the anticancer coordination compound. Students must specify the cis isomer. A question might give both cis and trans formulas and ask which is pharmacologically active.
Q. The anticancer drug cisplatin is
A. cis-[Pt(NH3)2Cl2] B. trans-[Pt(NH3)2Cl2] C. [Pt(NH3)4]Cl2 D. K2[PtCl4]
Trick: cis-[Pt(NH3)2Cl2] (Option A): Only the cis isomer can cross-link adjacent DNA bases because both Cl atoms are on the same side. Option B (trans) cannot achieve this geometry. Options C and D are different compounds entirely.