Inorganic Chemistry Chapters

Chapter summaries, must-read concepts, exam tips, tricks and IUPAC/VBT/CFT quick references for d&f Block Elements and Coordination Compounds.

🎯 Must-Read — These topics appear every year

Lanthanoid contraction — cause (poor 4f shielding), 3 consequences (Zr/Hf, 2nd/3rd row d-block similar, basicity of hydroxides)
Variable oxidation states — why d-block shows variable OS (incompletely filled d-orbitals); comparison with p-block
KMnO₄ reactions — acidic, neutral and alkaline medium; preparation from MnO₂
Magnetic properties — number of unpaired electrons determines paramagnetism; μ = √(n(n+2)) BM
Electronic configuration of 4th period transition metals — Cr([Ar]3d⁵4s¹) and Cu([Ar]3d¹⁰4s¹) exceptions
K₂Cr₂O₇ reactions — oxidising action in acidic medium; dichromate test for alcohol

📖 Chapter Summary — d-Block Characteristics

Property	Explanation	Example / Value
Definition	Elements in which last electron enters d-subshell (Groups 3–12)	Sc to Zn (1st row), Y to Cd (2nd), La to Hg (3rd)
General config	(n−1)d¹⁻¹⁰ ns¹⁻²	Exceptions: Cr (3d⁵4s¹), Cu (3d¹⁰4s¹)
Variable oxidation states	ns and (n−1)d electrons both used. All OS from +1 to highest possible	Mn: +2 to +7; Fe: +2, +3; Cu: +1, +2
Highest OS = group number	Up to Mn (group 7, max +7). After Mn, highest OS decreases.	Mn (+7), Cr (+6), V (+5), Ti (+4)
Metallic character	Hard, high MP/BP (except Hg), good conductors	W highest MP (3410°C); Hg lowest (−39°C)
Paramagnetism	Due to unpaired d-electrons. μ = √(n(n+2)) BM	Mn²⁺ (d⁵): 5 unpaired → μ = √35 ≈ 5.92 BM
Catalytic activity	Variable OS allows electron transfer; unsaturated d-orbitals adsorb reactants	Fe in Haber process; V₂O₅ in Contact process; Ni in hydrogenation
Colour of compounds	d–d electronic transition absorbs visible light → complementary colour seen	Cu²⁺ (d⁹) → blue; Mn²⁺ (d⁵) → faint pink; Zn²⁺ (d¹⁰) → colourless
Interstitial compounds	Small atoms (H, C, N) fit in gaps of metallic lattice → increase hardness	Steel = Fe + C interstitial; TiH₂ (hydrogen storage)
Alloy formation	Similar atomic sizes allow mixing in metallic lattice	Brass (Cu+Zn), Bronze (Cu+Sn), Stainless steel (Fe+Cr+Ni)

📐 Electronic Configs — Exceptions to Know

Cr (Z=24) — 3d⁵ 4s¹ (NOT 3d⁴ 4s²)

[Ar] 3d⁵ 4s¹

Half-filled 3d is extra stable (exchange energy maximum). Rule: half-filled and fully-filled d/f subshells are particularly stable.

Cu (Z=29) — 3d¹⁰ 4s¹ (NOT 3d⁹ 4s²)

[Ar] 3d¹⁰ 4s¹

Fully-filled 3d is extra stable. Cu²⁺ (3d⁹) is more stable than Cu⁺ (3d¹⁰) in aqueous solution because of high hydration enthalpy of Cu²⁺.

4th Period Transition Metal Ion Configs

Fe²⁺: [Ar] 3d⁶ (lose 4s first, then 3d) Fe³⁺: [Ar] 3d⁵ Mn²⁺: [Ar] 3d⁵ Cu²⁺: [Ar] 3d⁹

Rule: When forming cation from transition metal, always remove 4s electrons first, then 3d.

💡 Exam Tips & Tricks

🎯
Magnetic moment formula: μ = √(n(n+2)) BM where n = number of unpaired electrons. Memorise: 1 unpaired → 1.73 BM; 2 → 2.83; 3 → 3.87; 4 → 4.90; 5 → 5.92.
🎯
Colour rule — d¹⁰ and d⁰ are colourless: Zn²⁺ (d¹⁰), Sc³⁺ (d⁰), Ti⁴⁺ (d⁰) — all colourless because no d–d transition is possible. Any ion with partially filled d is coloured.
🎯
Why Mn shows maximum OS (+7): Mn has 7 electrons in outer shell (3d⁵4s²) → all 7 can participate in bonding → maximum OS = +7. Group number = maximum oxidation state (for groups 3–7).
🎯
Catalytic property reason: Two reasons ALWAYS needed: (1) Variable oxidation states — allows intermediate complex formation, acts as electron shuttle. (2) Large surface area of transition metals — reactants adsorb on surface.
🎯
Highest oxidation state in oxo-anions/fluorides: Mn shows +7 in MnO₄⁻; Cr shows +6 in Cr₂O₇²⁻ and CrO₄²⁻. Beyond Mn, fluorine and oxygen are needed to stabilise high OS (not possible with other ligands).
⚠️
Zn is NOT a transition metal: Zn has completely filled d-orbitals in ground state AND in common oxidation state (Zn²⁺: d¹⁰). No variable OS, no colour, no paramagnetism → not a true transition element. But it IS a d-block element.

🟣 KMnO₄ — Reactions in Different Media

Medium	Reaction with KMnO₄	Product of Mn	Colour change
Acidic (H₂SO₄)	MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O Oxidises FeSO₄, H₂C₂O₄, KI, H₂O₂, SO₂	Mn²⁺ (colourless / pale pink)	Purple → colourless
Neutral / faintly alkaline	MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻ Oxidises iodide to iodate	MnO₂ (brown ppt)	Purple → brown ppt
Strongly alkaline	MnO₄⁻ + e⁻ → MnO₄²⁻ Gains only 1 electron per Mn	MnO₄²⁻ (manganate, green)	Purple → green

🟠 KMnO₄ Important Reactions — Exam Ready

With FeSO₄ (acidic):

MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O

With H₂C₂O₄ (oxalic acid, acidic, warm):

2MnO₄⁻ + 5H₂C₂O₄ + 6H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O

💡 This reaction is self-indicating (purple disappears) and is used in permanganate titrations. The reaction is slow at first, then accelerates (autocatalysis by Mn²⁺ produced).

🟠 K₂Cr₂O₇ — Key Facts

Aspect	Detail
Colour & structure	Orange-red crystals; Cr in +6 OS; Cr₂O₇²⁻ ion has two CrO₄ tetrahedra sharing one O
Preparation	2K₂CrO₄ + H₂SO₄ → K₂Cr₂O₇ + 2KOH + H₂O (acidification of chromate)
Acidic medium reaction	Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (E° = +1.33V)
With FeSO₄	K₂Cr₂O₇ + 7H₂SO₄ + 6FeSO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 3Fe₂(SO₄)₃ + 7H₂O
With alcohol (test)	Orange K₂Cr₂O₇ + alcohol → green Cr³⁺. Used in breathalyser test.
Chromate ⇌ Dichromate	2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O (yellow chromate in base → orange dichromate in acid)

🔬 Lanthanoids & Lanthanoid Contraction

Aspect	Detail
Definition	Elements Ce (58) to Lu (71); last electron fills 4f subshell
Common OS	+3 is the most stable for all lanthanoids. Some show +2 (Eu, Sm) or +4 (Ce, Tb).
Lanthanoid contraction	Steady decrease in atomic/ionic radius from La→Lu due to poor shielding by 4f electrons → increasing Z_eff
Consequence 1	Zr (5th period) and Hf (6th period) have almost identical radii (~160 pm) → very difficult to separate
Consequence 2	2nd and 3rd row transition metals (e.g., Mo/W, Nb/Ta, Ru/Os) have similar sizes and properties
Consequence 3	Basicity of lanthanoid hydroxides decreases La(OH)₃ → Lu(OH)₃ (La is most basic)
Colour	Many lanthanoid ions are coloured due to f–f transitions. Lanthanoid series shows a characteristic colour pattern.

🔢 Magnetic Moment Quick Reference

Ion	Config	Unpaired e⁻	μ (BM)	Colour
Sc³⁺	3d⁰	0	0	Colourless
Ti³⁺	3d¹	1	1.73	Purple
V³⁺	3d²	2	2.83	Green
Cr³⁺	3d³	3	3.87	Green/violet
Mn²⁺ / Fe³⁺	3d⁵	5	5.92 (max)	Faint pink / yellow
Fe²⁺	3d⁶	4	4.90	Pale green
Cu²⁺	3d⁹	1	1.73	Blue
Zn²⁺	3d¹⁰	0	0	Colourless

🟠 K₂Cr₂O₇ — Preparation from Chromite Ore

Step	Reaction	Conditions
Step 1 — Roasting chromite ore	4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂	Chromite ore (iron chromite) roasted with Na₂CO₃ in excess air at high temperature → sodium chromate (yellow solution after leaching)
Step 2 — Acidification	2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O	Filtrate acidified with H₂SO₄ → CrO₄²⁻ (yellow) converted to Cr₂O₇²⁻ (orange) → sodium dichromate crystallises out
Step 3 — Conversion to K₂Cr₂O₇	Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl	Treatment with KCl → K₂Cr₂O₇ precipitates (less soluble than Na₂Cr₂O₇ at room temperature)
Chromate ⇌ Dichromate equilibrium	2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O	Yellow (chromate, alkaline) ⇌ Orange (dichromate, acidic). Shift with pH.

⚛️ Lanthanoids vs Actinoids — Full Comparison (NCERT Table)

Property	Lanthanoids (4f series, Ce–Lu)	Actinoids (5f series, Th–Lr)
Series position	Elements 58–71; 4f electrons filling	Elements 90–103; 5f electrons filling
Electronic configuration	[Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s²	[Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s²
Common oxidation state	+3 (most stable); few show +2 (Eu, Sm, Yb) or +4 (Ce, Tb, Pr)	+3 and +4 both common; wide range from +3 to +7 possible (5f, 6d, 7s all similar energy)
Oxidation state variety	Limited variable OS; mostly +3	Wide range of OS due to comparable 5f, 6d, 7s energies
Ionic radii	Gradual decrease La³⁺(106 pm) → Lu³⁺(86 pm) — lanthanoid contraction	Gradual decrease — actinoid contraction (greater than lanthanoid contraction due to poor shielding by 5f electrons)
Magnetic properties	Paramagnetic (unpaired 4f electrons); magnetic moments don't follow spin-only formula well	Paramagnetic; magnetic moments more complex (both spin and orbital contributions)
Colour of ions	Many are coloured (f–f transitions); colour pattern: Ce³⁺ colourless, Pr³⁺ green, Nd³⁺ lilac, Sm³⁺ yellow, Eu³⁺ pale pink, etc.	Many actinoid ions are also coloured; absorption spectra are sharper than lanthanoids
Tendency to form complexes	Less tendency to form complexes (lanthanoids are hard acids, prefer O-donor ligands)	Greater tendency (larger ionic size, higher OS available → more complex formation)
Radioactivity	Non-radioactive (except Pm — no stable isotopes)	All radioactive (all have unstable nuclei; Th, U naturally occurring)
Availability / occurrence	Found in nature (monazite, bastnäsite ores); misch metal alloy = La + Ce + Pr + Nd	Mostly synthetic (transuranium elements beyond U are man-made); Th and U occur naturally
Applications	Misch metal (lighter flints); CeO₂ (catalytic converters); Nd magnets; La in camera lenses; Eu in TV screens (red phosphor)	Th and U as nuclear fuels; Pu in nuclear weapons; Am in smoke detectors

❌ Common Mistakes to Avoid

Calling Zn a transition metal — it's d-block but NOT transition (d¹⁰ always)
Writing electron config: remove 4s before 3d when forming cation (Fe → Fe²⁺: lose 4s², not 3d²)
Saying lanthanoids don't show variable OS — they do (most common is +3, but Ce→+4, Eu→+2)
Giving only one reason for catalytic activity — always give TWO: variable OS + large surface area
In KMnO₄ reactions: product in acidic medium is Mn²⁺ (colourless/pale pink), NOT MnO₂
Confusing CrO₄²⁻ (yellow, alkaline) with Cr₂O₇²⁻ (orange, acidic) — they interconvert with pH
In K₂Cr₂O₇ preparation: chromite ore (FeCr₂O₄) is roasted with Na₂CO₃, not KOH. KCl is used only in the final step to precipitate K₂Cr₂O₇.
Actinoids showing +3 and +4 as common OS (not just +3 like lanthanoids) — because 5f, 6d, 7s are close in energy.

🎯 Must-Read — Every year, minimum 5 marks from these

IUPAC nomenclature rules — ligand names alphabetically, then metal, then OS in Roman numerals in brackets
VBT — hybridisation, geometry, magnetic character — draw d-orbital diagram; identify inner/outer orbital complex
Crystal Field Theory — d-orbital splitting in octahedral field; Δo; high-spin vs low-spin; colour explanation
Types of isomerism — structural (ionisation, linkage, coordination, solvate) and stereoisomerism (geometric, optical)
Werner's theory — primary valency (ionisable), secondary valency (non-ionisable), coordination number
EAN rule (Effective Atomic Number) — for stable complexes; n(electrons on metal) + n(electrons from ligands) = 36

📖 Werner's Theory — Quick Summary

Concept	Meaning	Example with [Co(NH₃)₆]Cl₃
Primary valency	Ionisable valency; satisfied by counter ions outside coordination sphere	3 Cl⁻ outside → primary valency = 3 → Co is Co³⁺
Secondary valency	Non-ionisable; satisfied by ligands directly bonded to metal	6 NH₃ inside → secondary valency = 6 → CN = 6
Coordination number	Total number of ligand donor atoms directly bonded to metal	CN = 6 (6 N atoms from NH₃)
Coordination sphere	Metal + ligands inside square brackets	[Co(NH₃)₆]³⁺
Counter ions	Ions outside coordination sphere; balance charge; ionise in solution	3 Cl⁻ (give 3 ions in solution)

🔤 IUPAC Nomenclature — Rules & Ligand Names

📋 Naming Rules (in order)

1
Cation before anion — name cation first (as with ionic compounds)
2
Ligands alphabetically — name all ligands in alphabetical order (ignore prefixes di-, tri- when alphabetising)
3
Metal name + OS — then name the central metal; OS in Roman numerals in parentheses e.g. iron(III)
4
Anionic complex — if complex is an anion, add "-ate" suffix to metal name. e.g. ferrate, chromate, cuprate, platinate
5
Bridging ligands — prefix μ (mu). e.g. μ-oxo for bridging O²⁻

📋 Key Ligand Names

Formula	Ligand name	Type
NH₃	ammine (double m)	neutral
H₂O	aqua	neutral
CO	carbonyl	neutral
NO	nitrosyl	neutral
Cl⁻	chlorido	anionic
CN⁻	cyanido	anionic
OH⁻	hydroxido	anionic
NO₂⁻	nitrito (N-bonded: nitro)	anionic
SCN⁻	thiocyanato (S); isothiocyanato (N)	anionic
en	ethane-1,2-diamine	bidentate
ox²⁻	oxalato	bidentate
EDTA⁴⁻	edta	hexadentate

🔤 IUPAC Naming — Worked Examples

[Co(NH₃)₆]Cl₃ → hexaamminecobalt(III) chloride
[PtCl₂(NH₃)₂] → diamminedichloridoplatinum(II) [Ligands: chlorido (c) before ammine (a)? No — alphabetical: ammine(a) before chlorido(c) → diamminedichloridoplatinum(II)]
[Fe(CN)₆]⁴⁻ → hexacyanidoferrate(II) ion ["ferrate" because complex anion]
[Co(en)₂Cl₂]⁺ → dichloridobis(ethane-1,2-diamine)cobalt(III) ion [bis- for bidentate ligands]
K₃[Fe(C₂O₄)₃] → potassium trisoxalatoferrate(III)

💡 Use di, tri, tetra for simple ligands; bis, tris, tetrakis for ligands whose names already contain a number (like en, EDTA, ox)

🔀 Types of Isomerism in Coordination Compounds

Ionisation Isomerism

Same formula but different ions inside/outside coordination sphere. Both compounds give different ions in solution.

[Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br

Linkage Isomerism

Ambidentate ligand (can bond through different atoms) is bonded through different atom in each isomer.

[Co(NH₃)₅NO₂]²⁺ (N-bonded, nitro) vs [Co(NH₃)₅ONO]²⁺ (O-bonded, nitrito)

Coordination Isomerism

In a salt where both cation and anion are complex ions — ligands are distributed differently between the two centres.

[Co(NH₃)₆][Cr(CN)₆] vs [Cr(NH₃)₆][Co(CN)₆]

Solvate (Hydrate) Isomerism

Different number of water molecules inside vs outside coordination sphere.

[Cr(H₂O)₆]Cl₃ vs [Cr(H₂O)₅Cl]Cl₂·H₂O vs [Cr(H₂O)₄Cl₂]Cl·2H₂O

Geometric (cis-trans) Isomerism

Same ligands arranged differently in space. Possible in square planar (MA₂B₂) and octahedral (MA₄B₂, MA₃B₃) complexes.

cis-[Pt(NH₃)₂Cl₂] (cisplatin, anticancer) vs trans-[Pt(NH₃)₂Cl₂]

Optical Isomerism

Non-superimposable mirror images (enantiomers). Rotate plane-polarised light. Common in octahedral complexes with bidentate ligands.

[Co(en)₃]³⁺ (Δ and Λ forms); cis-[CrCl₂(en)₂]⁺

🔬 VBT — Hybridisation Guide

[Co(NH₃)₆]³⁺ — Inner orbital, d²sp³

Co³⁺: [Ar] 3d⁶ Strong field (NH₃): electrons pair in 3d 3d: [↑↓][↑↓][↑↓][ ][ ] → 2 empty 3d orbitals available Hybridisation: d²sp³ (uses 2× inner 3d + 1× 4s + 3× 4p)

Geometry: Octahedral | Unpaired e⁻: 0

Diamagnetic

[CoF₆]³⁻ — Outer orbital, sp³d²

Co³⁺: [Ar] 3d⁶ Weak field (F⁻): electrons NOT paired in 3d 3d: [↑↓][↑][↑][↑][↑] → no empty 3d available Hybridisation: sp³d² (uses 4s + 3×4p + 2×4d)

Geometry: Octahedral | Unpaired e⁻: 4

Paramagnetic (μ = 4.90 BM)

[Ni(CO)₄] — sp³, tetrahedral

Ni⁰: [Ar] 3d¹⁰ Strong field CO: all 3d filled Hybridisation: sp³ Geometry: Tetrahedral

Unpaired e⁻: 0 | Special: zero oxidation state Ni

Diamagnetic

[Ni(CN)₄]²⁻ — dsp², square planar

Ni²⁺: [Ar] 3d⁸ Strong field CN⁻: one 3d pair moves to 4p 3d: [↑↓][↑↓][↑↓][↑↓][ ] → one 3d empty Hybridisation: dsp² (uses one 3d + one 4s + two 4p)

Geometry: Square planar | Unpaired e⁻: 0

Diamagnetic

🔬 VBT Decision Flowchart

Step 1: Write electronic config of metal ion (remove electrons from 4s first)

Step 2: Identify ligand strength — Strong field: CO, CN⁻, NH₃, en, NO₂⁻ | Weak field: F⁻, Cl⁻, Br⁻, I⁻, H₂O

Step 3: Strong field → electrons pair in d → check if inner d-orbitals free → d²sp³ (octahedral) or dsp² (square planar)

Step 4: Weak field → electrons stay unpaired → use outer 4d → sp³d² (octahedral) or sp³ (tetrahedral)

Step 5: Count unpaired electrons → calculate μ = √(n(n+2)) BM → state diamagnetic (n=0) or paramagnetic

💎 Crystal Field Theory (CFT)

📐 d-Orbital Splitting

• t₂g (3 orbitals): 0.4Δo below avg — lower energy
• eg (2 orbitals): 0.6Δo above avg — higher energy
• Large Δo → Strong field → low spin (electrons pair in t₂g)
• Small Δo → Weak field → high spin (electrons go to eg)

💡 CFT Exam Tips

🎯
Spectrochemical series — memorise this sequence:
I⁻ < Br⁻ < S²⁻ < SCN⁻ < Cl⁻ < NO₃⁻ < F⁻ < OH⁻ < H₂O < NCS⁻ < en < NH₃ < NO₂⁻ < CN⁻ < CO
Weak field ←————————————————→ Strong field
🎯
Colour from CFT: Δo corresponds to visible light. Electron absorbs this light to jump t₂g → eg. Complementary colour is what we see. Example: [Ti(H₂O)₆]³⁺ absorbs green → looks violet/purple.
🎯
CFSE (Crystal Field Stabilisation Energy): For d⁶ low spin — 6 electrons in t₂g: CFSE = 6(−0.4Δo) = −2.4Δo. For d⁶ high spin — 4 in t₂g + 2 in eg: CFSE = 4(−0.4Δo) + 2(+0.6Δo) = −0.4Δo. Low spin = more stable.
🎯
d¹⁰ complexes have no CFSE and are often colourless (no d–d transition possible). [Zn(H₂O)₆]²⁺ is colourless.
⚠️
VBT vs CFT: VBT predicts hybridisation and geometry but cannot fully explain colour or exact magnetic behaviour. CFT explains colour and distinguishes high-spin/low-spin better. CBSE may ask for either — read the question.

🌐 Applications of Coordination Compounds

Application	Compound	Detail
Anticancer drug	cis-Platin [Pt(NH₃)₂Cl₂]	Geometric isomer; binds to DNA; inhibits cell division
Extraction of silver/gold	[Ag(CN)₂]⁻ / [Au(CN)₂]⁻	Hydrometallurgy: ore + NaCN solution → complex → Zn displaces metal
Electroplating	[Ag(CN)₂]⁻ bath	Provides slow, controlled release of Ag⁺ for uniform plating
EDTA in medicine	[EDTA-Ca]²⁻	Lead poisoning treatment — EDTA chelates Pb²⁺, excreted in urine
Haemoglobin	Fe(II) porphyrin complex	O₂ transport; CO binds more strongly → CO poisoning
Chlorophyll	Mg(II) porphyrin complex	Photosynthesis
Vitamin B₁₂	Co(III) corrin complex	First naturally occurring organometallic compound

🔢 Naming Practice — Quick Test

[CoCl₃(NH₃)₃] = triamminetrichloridocobalt(III)
K₂[PtCl₄] = potassium tetrachloridoplatinate(II)
[Cr(en)₃]³⁺ = tris(ethane-1,2-diamine)chromium(III) ion
[Fe(CO)₅] = pentacarbonyliron(0)
Na₂[Fe(CN)₅NO] = sodium pentacyanidoitrosoylferrate(II)

💡 For complex anions (inside square bracket negative): metal gets "-ate" suffix. Check common Latin names: Fe → ferrate, Cu → cuprate, Ag → argentate, Au → aurate, Pb → plumbate, Sn → stannate

❌ Common Mistakes to Avoid

Naming ligands in formula order instead of alphabetical order — always alphabetical
Using "ammine" (coordination compound NH₃) vs "amine" (organic compound RNH₂) — double-m for complex
Forgetting to use "bis/tris" for complex ligand names (en, ox) — NOT "di/tri"
In VBT: forgetting to remove 4s electrons first from metal before checking d configuration for ion
Saying high-spin = more stable — WRONG. Low-spin has higher CFSE → more stable (for most cases)
Optical isomerism in tetrahedral — ONLY with 4 different ligands; it's rare. Square planar usually doesn't show optical isomerism (planar, mirror image is superimposable).
Confusing inner orbital (d²sp³) with outer orbital (sp³d²) — inner uses 3d (inside), outer uses 4d (outside)

⚖️ Stability of Coordination Compounds

Concept	Detail
Stability constant (K_f or K_stab)	Equilibrium constant for the formation of a complex ion from its components. Larger K_f → more stable complex. M^n+ + xL ⇌ [MLₓ]^n+ K_f = [[MLₓ]^n+] / ([M^n+][L]^x)
Instability constant (K_d)	Reciprocal of K_f. Represents dissociation of complex. K_d = 1/K_f. Smaller K_d → more stable (less tendency to dissociate).
Stepwise formation constants	For [Cu(NH₃)₄]²⁺: ligands added one by one, each step has its own K₁, K₂, K₃, K₄. Overall K_f = K₁ × K₂ × K₃ × K₄. Stepwise constants generally decrease (each successive ligand harder to add due to steric/electrostatic reasons).
Factors affecting stability	1. Nature of metal ion: Higher charge density (charge/radius) → stronger attraction for ligands → more stable. Fe³⁺ > Fe²⁺. 2. Nature of ligand: Strong field ligands (CN⁻, en, EDTA) → more stable. Chelating ligands (EDTA, en) → much more stable than monodentate (chelate effect). 3. Chelate effect: Polydentate ligands form more stable complexes than monodentate ligands of comparable donor strength (entropy-driven — more ligand molecules released from solvent cage when chelate forms).
EDTA complexes	EDTA (ethylenediaminetetraacetate) is a hexadentate ligand — forms extremely stable complexes (very large K_f). Used in: water softening (removes Ca²⁺, Mg²⁺), analytical chemistry (complexometric titrations), as antidote to heavy metal poisoning (Pb²⁺, Hg²⁺ poisoning).
Cisplatin [PtCl₂(NH₃)₂]	Square planar Pt(II) complex. cis-isomer is an anticancer drug (binds to DNA, blocks replication of cancer cells). trans-isomer is inactive. Used in treatment of testicular, ovarian, bladder cancer.
Wilkinson's catalyst [RhCl(PPh₃)₃]	Rhodium complex — used as homogeneous catalyst for hydrogenation of alkenes (adds H₂ across C=C at room temperature, atmospheric pressure). Nobel Prize (Wilkinson, 1973).
Biological coordination compounds	Haemoglobin: Fe²⁺ in porphyrin ring (haem); O₂ binds reversibly at 6th coordination site. Chlorophyll: Mg²⁺ in porphyrin ring; central to photosynthesis. Vitamin B₁₂: Co³⁺ complex; essential for red blood cell formation. Carbonic anhydrase: Zn²⁺ complex; catalyses CO₂ ⇌ HCO₃⁻ in blood.

🎯 Applications Summary — Memory Card

EDTA: Hexadentate → extremely stable → used in water softening, medicine (antidote), analytical titrations.

Cisplatin: cis = cancer drug; trans = inactive. Pt(II), square planar, [PtCl₂(NH₃)₂].

Bio complexes mnemonic — Fe, Mg, Co, Zn: Fe (Hb, blood), Mg (chlorophyll, green), Co (B₁₂, blood cells), Zn (enzyme, carbonic anhydrase).

Chelate effect: More teeth = more stable. EDTA (6 teeth) > en (2 teeth) > NH₃ (1 tooth). Driven by entropy increase.