For JEE Main and NEET aspirants, Class 12 Chemistry Chapter 5 Coordination Compounds is the single most reliable inorganic chapter, delivering four to five guaranteed questions across every paper since 2021 on IUPAC nomenclature, crystal field splitting, and isomerism. This revision page hosts the 28-page Notes PDF, the sub-topic weightage map, and the full crystal-field theory walk-through.
- CBSE Boards: 6 to 8 marks every year, typically one 3-mark IUPAC-naming question, one 2-mark hybridisation or magnetic-moment problem, and one assertion-reason MCQ on isomerism.
- JEE Main: 4 to 5% of the Chemistry paper, with 2 to 3 questions per shift on CFSE, geometrical and optical isomerism, and the spectrochemical series.
- NEET: 3 to 4 questions per year, the highest contribution from any inorganic chapter, mostly on coordination number, EAN, and ligand-field strength.
The notes below build the chapter the way examiners actually mark it: Werner's postulates and the primary-versus-secondary valence distinction first, then IUPAC nomenclature with worked names, then isomerism with structural diagrams, and finally the three bonding theories (VBT, CFT, and a brief LFT note) with magnetic moment and colour applications.
These Collegedunia Coordination Compounds notes are curated by subject experts, mapped to the 2026-27 NCERT print, and refined against the last five years of CBSE Board, JEE Main, and NEET papers.
Also Check:
- Coordination Compounds Class 12 Chemistry NCERT Solutions
- Coordination Compounds Class 12 Chemistry Formula Sheet
- CBSE Class 12 Chemistry Syllabus 2026-27
Key Topics Covered in Coordination Compounds Class 12 Notes
The notes are organised so the chapter's most-Googled sub-topics are revised in their CBSE / JEE / NEET sequence. The chip list below maps every search-driven concept to its place in the PDF.
Coordination Compounds Video Walkthrough
Source: Magnet Brains on YouTube
Why Coordination Compounds Matter for CBSE, JEE and NEET
This is the chapter where descriptive inorganic chemistry finally meets a real theoretical framework. A coordination compound is a structure in which a central metal atom or ion is surrounded by a fixed number of neutral or anionic ligands bonded to it through coordinate (dative) covalent bonds. The chapter rewards three skills together: naming complexes correctly under IUPAC rules, predicting their geometry and isomerism, and explaining their magnetic and optical behaviour using crystal field theory. CBSE, JEE Main, and NEET all draw heavily from this dependence map.
The chapter is also the gateway to biological inorganic chemistry: haemoglobin (Fe-porphyrin), chlorophyll (Mg-porphyrin), vitamin B12 (Co-corrin), and EDTA-metal therapy are all coordination complexes. NEET regularly tests these life-science applications because they connect cleanly to Biology.
How will Collegedunia's NCERT Notes Help You with Coordination Compounds?
These notes are written by inorganic-chemistry specialists who have decoded the last five years of CBSE marking schemes and the 2024 and 2025 JEE Main and NEET shift papers, so what you read here is what gets ticked on the answer sheet.
- 2026-27 NCERT Alignment: Every topic matches the current 2026-27 syllabus, including the retained crystal field theory section, the IUPAC 2005 nomenclature update, and the trimmed bonding-theory coverage.
- Worked IUPAC Names: Twenty complexes are renamed step by step, because the chapter's biggest mark loss comes from cation-anion order, ligand alphabetisation, and oxidation-state brackets.
- CFT Splitting Diagrams in Colour: Octahedral and tetrahedral d-orbital splitting drawn with energy levels labelled, so the difference between high-spin and low-spin is visual, not just verbal.
- Expert Verification: Every magnetic moment, geometry, and oxidation state is cross-checked against the latest 2026-27 NCERT textbook print and the most recent JEE Main official answer key (2025 cycle).
Most Important Sub-Topics in Coordination Compounds and Their Mark Distribution
An audit of CBSE, JEE Main and NEET papers from 2021 to 2025 shows the chapter's marks concentrate on a tight set of sub-topics. The table below ranks them by frequency of appearance so revision time can be apportioned correctly.
| Sub-topic | Weightage | CBSE Frequency |
|---|---|---|
| IUPAC nomenclature of mononuclear complexes | High | Almost every year |
| Crystal field theory: octahedral splitting and CFSE | High | Almost every year |
| Isomerism (geometrical, optical, linkage, ionisation) | High | 4 out of last 5 years |
| Hybridisation, geometry and magnetic moment (VBT) | High | 4 out of last 5 years |
| Coordination number, EAN, and Werner postulates | Medium | 3 out of last 5 years |
| Stability constants and chelate effect | Medium | 3 out of last 5 years |
| Spectrochemical series and strong vs weak ligands | Medium | 3 out of last 5 years |
| Bonding in metal carbonyls (synergic effect) | Low | 2 out of last 5 years |
| Importance and applications in biology and medicine | Low | 2 out of last 5 years |
Coordination Compounds Topic-by-Topic Notes for Class 12 Chemistry
Werner's framework defines the vocabulary, IUPAC rules formalise the names, isomerism reveals the structural variety, and bonding theories explain magnetism and colour. Each H3 below follows that order.
Werner's Theory and Core Definitions
Alfred Werner proposed in 1893 that every coordination compound has two valences: primary (ionisable) equal to the oxidation state of the central metal, and secondary (non-ionisable) equal to the coordination number. The ligands occupy the secondary valences and lie inside the coordination sphere, shown in square brackets. Key terms locked in for the rest of the chapter:
- Coordination sphere: the central metal plus its ligands, written in square brackets, e.g. [Co(NH3)6]3+.
- Coordination number (CN): the number of donor atoms directly bonded to the metal. Common values are 4 (tetrahedral or square planar) and 6 (octahedral).
- Denticity: the number of donor sites on a single ligand. Monodentate (NH3, Cl-), bidentate (en, ox2-), hexadentate (EDTA4-).
- Chelate: a complex in which a polydentate ligand forms a ring with the central metal, giving extra stability (chelate effect).
- EAN (Effective Atomic Number): total electrons on the metal after coordination; equals Z - oxidation state + 2 × CN .
IUPAC Nomenclature: The 2005 Rules Applied Step by Step
Naming follows a fixed sequence. Cation is named first, anion second. Inside a complex ion: ligands come before the metal, in alphabetical order, with their counts shown as di-, tri-, tetra- (or bis-, tris- for ligands that already contain numerical prefixes like ethylenediamine). The oxidation state of the metal is given in Roman numerals in parentheses immediately after the metal name. Anionic ligands end in -o (chloro, hydroxo, sulphato); neutral ligands keep their name (ammine for NH3, aqua for H2O, carbonyl for CO).
For an anionic complex, the metal name takes the suffix -ate (cuprate for Cu, ferrate for Fe, plumbate for Pb, cobaltate for Co). For a cationic or neutral complex, the metal keeps its English name. Worked names:
| Formula | IUPAC Name |
|---|---|
| [Co(NH3)6]Cl3 | hexaamminecobalt(III) chloride |
| K4[Fe(CN)6] | potassium hexacyanidoferrate(II) |
| [Pt(NH3)2Cl2] | diamminedichloridoplatinum(II) |
| [Cr(H2O)4Cl2]Cl | tetraaquadichloridochromium(III) chloride |
| Na3[Co(NO2)6] | sodium hexanitrito-N-cobaltate(III) |
Isomerism in Coordination Compounds
Coordination compounds show two broad categories: structural isomerism (different atom connectivities) and stereoisomerism (same connectivity, different spatial arrangement).
- Ionisation isomerism: exchange of ligand between the sphere and the counter-ion, e.g. [Co(NH3)5SO4]Br vs [Co(NH3)5Br]SO4.
- Linkage isomerism: ambidentate ligand bonds through different donor atoms, e.g. -NO2 (nitrito-N) vs -ONO (nitrito-O).
- Coordination isomerism: exchange of ligands between two metal centres in a salt, e.g. [Co(NH3)6][Cr(CN)6] vs [Cr(NH3)6][Co(CN)6].
- Geometrical (cis-trans) isomerism: seen in square planar MA2B2 and octahedral MA4B2 systems. Cisplatin (cis-[Pt(NH3)2Cl2]) is anticancer; the trans isomer is not.
- Optical isomerism: chiral complexes that rotate plane-polarised light. [Co(en)3]3+ shows two non-superimposable mirror images (Δ and Λ).
Valence Bond Theory (VBT): Hybridisation, Geometry and Magnetism
VBT treats the metal-ligand bond as a coordinate covalent bond formed by ligand-lone-pair donation into hybrid metal orbitals. The hybridisation pattern determines geometry:
| Coordination Number | Hybridisation | Geometry | Example |
|---|---|---|---|
| 4 | sp3 | Tetrahedral | [NiCl4]2- |
| 4 | dsp2 | Square planar | [Ni(CN)4]2-, [Pt(NH3)2Cl2] |
| 6 (inner orbital, low spin) | d2sp3 | Octahedral | [Co(NH3)6]3+, [Fe(CN)6]3- |
| 6 (outer orbital, high spin) | sp3d2 | Octahedral | [CoF6]3-, [Fe(H2O)6]2+ |
The spin-only magnetic moment is μ = √n(n+2) BM, where n is the number of unpaired d-electrons. Strong-field ligands (CN-, CO) force electron pairing and give low-spin complexes; weak-field ligands (F-, H2O) allow high-spin complexes.
Crystal Field Theory (CFT) and CFSE
CFT treats ligands as point negative charges that split the degenerate metal d-orbitals into two sets. In an octahedral field, the d-orbitals split into a lower-energy t2g set (dxy, dyz, dzx) and a higher-energy eg set (dz2, dx2-y2). The energy gap is the crystal field splitting parameter o . In a tetrahedral field, the splitting is reversed and smaller: t = 49o .
CFSE = [ -0.4 n(t2g) + 0.6 n(eg) ]o + P
where n(t2g) and n(eg) are electron counts in each set and P is the pairing-energy correction for low-spin configurations.
The spectrochemical series ranks ligands by field strength:
I- < Br- < Cl- < F- < OH- < H2O < NH3 < en < CN- < CO. When o > P , pairing occurs and the complex is low-spin; when o < P , the complex is high-spin. This single inequality explains why [Fe(CN)6]4- is diamagnetic while [Fe(H2O)6]2+ is paramagnetic.
Bonding in Metal Carbonyls: The Synergic Effect
Metal carbonyls such as Ni(CO)4, Fe(CO)5, and Cr(CO)6 involve a special synergic bond: the CO lone pair donates into an empty metal hybrid orbital ( σ -donation), and the filled metal d-orbital donates back into the empty π* anti-bonding orbital of CO ( π -back-bonding). The back-bond strengthens the M-C bond and weakens the C-O bond, explaining the lower C-O stretching frequency in carbonyls compared to free CO.
Importance and Applications in Biology, Medicine and Industry
Coordination compounds run biology: haemoglobin is an iron-porphyrin complex that binds O2 reversibly; chlorophyll is the magnesium analogue that drives photosynthesis; vitamin B12 is a cobalt-corrin complex. Cisplatin ([Pt(NH3)2Cl2], cis) is a standard anticancer drug; the trans isomer is therapeutically inactive. EDTA chelates lead and mercury in heavy-metal poisoning. Industrially, Wilkinson's catalyst RhCl(PPh3)3 drives hydrogenation; Ni(CO)4 is used in the Mond process for nickel purification.
Coordination Compounds Top 6 Formulae for Quick Recall
The six lines below are the formulae you will use most often in CBSE and JEE Main numericals on this chapter. The complete master sheet with every CFSE configuration and stability-constant expression sits on the dedicated Collegedunia Formula Sheet for this chapter.
| Quantity / Concept | Expression |
|---|---|
| Effective Atomic Number | EAN = Z - ox. state + 2 × CN |
| Spin-only magnetic moment | μ = √n(n+2) BM |
| CFSE (octahedral) | CFSE = [-0.4 n(t2g) + 0.6 n(eg)]o + P |
| Tetrahedral splitting | t = 49 o |
| Overall stability constant | n = [MLn][M][L]n |
| Wavelength-energy relation (d-d transition) | o = hcλ |
Full master table: Coordination Compounds Class 12 Chemistry Formula Sheet
Common Misconceptions Students Hold in 12th Chemistry Chapter 5
These four wrong beliefs cause most of the dropped marks on this chapter. Address them before the board paper and Coordination Compounds becomes a steady 7-mark scorer.
- Confusing oxidation state with coordination number. Oxidation state is the charge on the central metal after removing all ligands with their lone pairs; coordination number counts the donor atoms attached. In K4[Fe(CN)6], Fe is in +2 (oxidation state) but with CN of 6. A standard 1-mark loss in nomenclature questions every year.
- Naming ligands by formula order instead of alphabetical order. IUPAC mandates alphabetical order of ligand names, ignoring numerical prefixes. So "ammine" comes before "chlorido", even when there are two chloridos and one ammine. CBSE 2024 deducted half a mark for incorrect ordering on a 3-mark question.
- Treating all bidentate ligands as chelates without checking ring size. Chelates need a 5- or 6-membered ring for stability. Ligands like en and oxalate form stable five-membered rings; longer chains rarely chelate effectively.
- Applying CFT without checking pairing energy. The high-spin versus low-spin choice depends on whether o exceeds P . Students often default to "strong ligand means low spin" without checking the metal's oxidation state, which is wrong for 3d complexes with d4 to d7 configurations.
Class 12 Chemistry Chapter 5 Previous Year Questions Snapshot
A quick year-wise scan of where Coordination Compounds surfaced across CBSE, JEE Main and NEET in the last six cycles. The fuller year-wise question map with topic tagging lives on the NCERT Solutions page for this chapter.
| Year | CBSE Board | JEE Main | NEET |
|---|---|---|---|
| 2026 | - | CFSE of [Fe(CN)6]3- | Pending (exam rescheduled) |
| 2025 | IUPAC name of [Co(en)2Cl2]+ (2M) | Optical isomerism of [Cr(ox)3]3- | Hybridisation in [Ni(CN)4]2- |
| 2024 | Geometrical isomers of [Pt(NH3)2Cl2] (3M) | Spin-only moment of [CoF6]3- | Spectrochemical series ranking |
| 2023 | Werner's theory postulates (3M) | Linkage isomerism with NO2- | EAN of Ni(CO)4 |
| 2022 | Crystal field splitting in octahedral (5M) | Chelate effect and stability | Magnetic moment of [Fe(H2O)6]2+ |
| 2021 | IUPAC nomenclature of three complexes (3M) | - | Coordination number and CFT |
Full year-wise PYQ map: Coordination Compounds Class 12 Chemistry NCERT Solutions
Frequently Asked Coordination Compounds Questions in CBSE Board Exams (2021 to 2026)
The three patterns below recur almost every year in CBSE Class 12 Chemistry. Practise the reasoning behind each, not just the surface facts.
Ques. Write the IUPAC name of [Co(NH3)5Cl]Cl2 and identify the oxidation state of Co. (CBSE 2021, 2025)
[2-Mark Question] Name: pentaamminechloridocobalt(III) chloride. Oxidation state of Co: let it be x. Net charge of complex ion = +2 (since two Cl- ions are outside), and NH3 is neutral while Cl- inside the sphere is -1. Therefore x + 5(0) + (-1) = +2, giving x = +3.
Ques. Draw the geometrical isomers of [Pt(NH3)2Cl2] and state which is biologically active. (CBSE 2024)
[3-Mark Question] The square planar complex has two isomers: cis (both Cl on adjacent corners, both NH3 on the other two adjacent corners) and trans (Cl on opposite corners). The cis isomer is cisplatin, an anticancer drug; the trans isomer is therapeutically inactive because it cannot form the correct DNA-Pt cross-links.
Ques. Using CFT, predict the geometry, hybridisation and magnetic moment of [Fe(CN)6]4-. (CBSE 2022, 2023)
[3-Mark Question] Fe is in +2 (d6). CN- is a strong-field ligand, so o > P and all six d-electrons pair into t2g. Hybridisation is d2sp3 (inner orbital), geometry is octahedral, number of unpaired electrons = 0, so μ = 0 BM. The complex is diamagnetic.
Coordination Compounds Class 12: Glossary of Must-Know Terms
These nine terms appear in CBSE one-mark MCQs and assertion-reason questions almost every year. Lock in the one-line definition for each before attempting the PYQ set.
| Term | One-line definition |
|---|---|
| Coordination compound | Compound in which a central metal is bonded to ligands via coordinate covalent bonds. |
| Ligand | An ion or molecule with at least one lone pair that donates electrons to the central metal. |
| Denticity | The number of donor atoms a single ligand uses to bond to the central metal. |
| Chelate | A ring complex formed when a polydentate ligand binds the same metal at two or more sites. |
| Coordination number | The number of ligand donor atoms directly bonded to the central metal. |
| EAN | Effective atomic number: total electrons on the metal after coordination. |
| Spectrochemical series | An empirical ranking of ligands by their crystal-field splitting strength. |
| CFSE | The net energy lowering of d-electrons after crystal-field splitting. |
| Ambidentate ligand | A ligand that can bond through either of two different donor atoms (e.g. NO2- / -ONO). |
Class 12 Chemistry Chapter 5 Weightage Compared Across All Chapters
The visual below maps the typical CBSE marks distribution across all 10 chapters of the Class 12 Chemistry NCERT, averaged over the last five board papers. Coordination Compounds shares the top tier with Solutions at 7 marks, ahead of every other inorganic and organic chapter.
Related Links:
- Coordination Compounds Class 12 Chemistry Handwritten Notes
- Coordination Compounds Class 12 Chemistry Exemplar Solutions
- CBSE Class 12 Chemistry Sample Papers 2026
More Coordination Compounds Chemistry Class 12 Resources
- Coordination Compounds Class 12 Chemistry NCERT Solutions
- Coordination Compounds Class 12 Chemistry Formula Sheet
- Coordination Compounds Class 12 Chemistry NCERT Book PDF
- Coordination Compounds Class 12 Chemistry NCERT Exemplar Book PDF
- Coordination Compounds Class 12 Chemistry NCERT Exemplar Solutions
- Coordination Compounds Class 12 Chemistry Handwritten Notes
NCERT Notes for Class 12 Chemistry: All Chapters
Cycle through any of the other nine chapters of the Class 12 Chemistry NCERT below. Each link opens the chapter's full revision notes page.
| Chapter | Resource |
|---|---|
| Chapter 1 | Solutions Notes |
| Chapter 2 | Electrochemistry Notes |
| Chapter 3 | Chemical Kinetics Notes |
| Chapter 4 | The d- and f-Block Elements Notes |
| Chapter 6 | Haloalkanes and Haloarenes Notes |
| Chapter 7 | Alcohols, Phenols and Ethers Notes |
| Chapter 8 | Aldehydes, Ketones and Carboxylic Acids Notes |
| Chapter 9 | Amines Notes |
| Chapter 10 | Biomolecules Notes |
Coordination Compounds Class 12 Chemistry Notes FAQs
Ques. Where can I download Coordination Compounds Class 12 Chemistry Notes PDF?
Ans. You can download the Coordination Compounds Class 12 Chemistry Notes PDF directly from this page. Both the Normal and HD versions are available, and both are free.
Ques. Are these notes aligned with the 2026-27 NCERT for Class 12 Chemistry?
Ans. Yes. The notes reflect the current 2026-27 syllabus for Class 12 Chemistry. The new NCERT edition retains all major sub-topics of Coordination Compounds, including Werner's theory, IUPAC nomenclature, isomerism, valence bond theory, and crystal field theory.
Ques. How many pages is the Class 12th Chemistry Coordination Compounds Notes PDF?
Ans. The Notes PDF runs approximately 28 pages and covers Werner's theory, ligands and denticity, coordination number and EAN, IUPAC nomenclature, structural and stereoisomerism, valence bond theory with hybridisation, crystal field theory with octahedral and tetrahedral splitting, bonding in metal carbonyls, and biological applications.
Ques. How much weightage does Coordination Compounds carry in Class 12 Chemistry Board Exam 2026?
Ans. Coordination Compounds carries 6 to 8 marks in the CBSE Class 12 Chemistry Board paper. The standard split is one 3-mark question on IUPAC nomenclature or geometrical isomerism, one 2-mark question on hybridisation or magnetic moment, plus one assertion-reason MCQ.
Ques. What is crystal field theory and how does it explain the colour of coordination compounds?
Ans. Crystal field theory treats ligands as point negative charges that split the metal's degenerate d-orbitals into two sets. In an octahedral field, the t2g set lies below the eg set by an energy gap o . A d-electron absorbs a visible-light photon of energy o and jumps from t2g to eg; the colour observed is complementary to the absorbed wavelength.
Ques. How do I write the IUPAC name of a coordination compound?
Ans. Name the cation first, then the anion. Inside a complex ion, name the ligands in alphabetical order with prefixes (di, tri) for count, then the metal name with its oxidation state in Roman numerals. Use -ate suffix on the metal if the complex ion is anionic. Example: K4[Fe(CN)6] is potassium hexacyanidoferrate(II).
Ques. What is the difference between high-spin and low-spin complexes?
Ans. If the crystal field splitting energy o is greater than the electron pairing energy P , electrons pair up in the lower t2g set first, giving a low-spin (inner-orbital, d2sp3) complex. If o < P , electrons stay unpaired across t2g and eg, giving a high-spin (outer-orbital, sp3d2) complex. Strong-field ligands like CN- and CO favour low spin; weak-field ligands like F- and H2O favour high spin.
Ques. Which sub-topics should I revise the night before the Class 12 Chemistry board exam?
Ans. Revise five sub-topics: IUPAC nomenclature of at least ten worked complexes, crystal field splitting in octahedral and tetrahedral fields with CFSE calculation, geometrical and optical isomerism of square planar and octahedral complexes, hybridisation and magnetic moment using VBT, and the spectrochemical series for predicting high-spin versus low-spin behaviour.
Ques. What is the difference between primary valence and secondary valence in Werner's theory?
Ans. Primary valence equals the oxidation state of the central metal, is ionisable, and is satisfied by anions outside the coordination sphere. Secondary valence equals the coordination number, is non-ionisable, is satisfied by ligands inside the sphere (square brackets), and is directional - it fixes the geometry. In [Co(NH3)5Cl]Cl2, Co3+ has primary valence 3 (the three Cl- total) and secondary valence 6 (five NH3 + one inner Cl-).
Ques. How does crystal field theory differ from valence bond theory (VBT vs CFT)?
Ans. VBT treats the metal-ligand bond as a coordinate covalent bond formed by ligand lone-pair donation into hybridised metal orbitals (sp3, dsp2, d2sp3, sp3d2) and predicts geometry plus magnetic behaviour. CFT treats the interaction as electrostatic, splits the d-orbitals into t2g and eg sets in an octahedral field by o , and quantitatively explains colour (d-d transitions), CFSE, magnetic moment, and the spectrochemical series. CFT is the modern framework; VBT is still used for hybridisation labels and EAN reasoning.
Ques. What is the EAN rule and how do I apply it to Ni(CO)4?
Ans. Sidgwick's Effective Atomic Number rule: EAN = Z(M) - oxidation state + 2 × CN, with extra stability when EAN equals the atomic number of the nearest noble gas. For Ni(CO)4, Z(Ni) = 28, oxidation state of Ni = 0, CN = 4, so EAN = 28 - 0 + 8 = 36, matching Kr. The same EAN = 36 is reached by [Fe(CN)6]4-, justifying its diamagnetism. Ni(CO)4 uses sp3 hybridisation, tetrahedral geometry, and is diamagnetic.
Ques. Why is the splitting in tetrahedral complexes smaller than in octahedral ( t vs o )?
Ans. In a tetrahedral field there are only 4 ligands (vs 6 in octahedral) and none of them sits directly along the d-orbital lobes; the ligand-orbital approach is between the lobes. The geometric factor gives t = 49 o ≈ 0.45 o . Because t is almost always smaller than the pairing energy P, tetrahedral complexes are essentially always high-spin. This is the standard 1-mark CFT MCQ on JEE Main.
Ques. What are ambidentate ligands and which linkage isomers do they form?
Ans. An ambidentate ligand has two donor atoms but bonds to the metal through only one at a time, generating linkage isomers. Classic NCERT pairs: NO2- bonded through N (nitrito-N, -NO2) vs through O (nitrito-O, -ONO); SCN- through S (thiocyanato-S, -SCN) vs through N (thiocyanato-N, -NCS); CN- through C (cyanido) vs through N (isocyanido). The two isomers usually differ in colour, stability, and field strength (CN- through C is a strong-field ligand, isocyanido is weaker).
Ques. Why is the chelate effect important and why is EDTA a hexadentate ligand?
Ans. The chelate effect is the extra thermodynamic stability of complexes with polydentate (chelating) ligands compared to monodentate-only analogues; it is entropy-driven (releasing free water molecules raises disorder). EDTA4- has six donor atoms - four carboxylate oxygens and two amine nitrogens - making it hexadentate. It wraps around a single metal centre forming five fused chelate rings, which is why it sequesters Pb2+, Hg2+ and Ca2+ so strongly that it is used in heavy-metal poisoning therapy and complexometric titrations.
Ques. Which coordination compounds are important in biology - haemoglobin, chlorophyll and Vitamin B12?
Ans. Haemoglobin is an Fe2+-porphyrin complex inside the globin protein; the iron centre binds O2 reversibly as its sixth ligand, enabling oxygen transport. Chlorophyll is the Mg2+-porphyrin (chlorin) analogue inside thylakoids that drives photosynthesis. Vitamin B12 (cyanocobalamin) is a Co3+-corrin complex with CN- as the sixth ligand; deficiency causes pernicious anaemia. NEET asks one biological-coordination question every cycle, most often on the metal centre + ring identity.
Ques. Why is cisplatin biologically active and the trans isomer not?
Ans. Cisplatin is cis-[Pt(NH3)2Cl2], a square planar Pt(II) complex with the two Cl- ligands on adjacent corners. Inside the cell the two Cl- ligands are displaced by water then by adjacent purine bases (G-N7 of DNA), creating a cis 1,2-intrastrand cross-link that distorts DNA and triggers apoptosis. The trans isomer (with Cl- at opposite corners) cannot form the same cis cross-link, so it is therapeutically inactive. This is the textbook example of geometrical isomerism dictating pharmacology.
Ques. What is the synergic bonding (back-bonding) in metal carbonyls like Ni(CO)4?
Ans. Metal carbonyls have a special two-way bond. (1) The lone pair on the C atom of CO donates into an empty metal hybrid orbital, forming a σ bond. (2) A filled metal d-orbital donates electron density back into the empty π* anti-bonding orbital of CO, forming a π back-bond. The back-donation strengthens the M-C bond and weakens the C-O bond, which is why the C-O stretching frequency drops from 2143 cm-1 in free CO to roughly 2050 cm-1 in Ni(CO)4. Ni(CO)4 is sp3 hybridised, tetrahedral, and diamagnetic.
Comments