Coordination Compounds Notes for NEET – Complete Chapter

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INTRODUCTION & ADDITION COMPOUNDS

Addition Compounds

Compounds made up of two or more stable salts in a fixed stoichiometric ratio by crystallisation.

Reaction: Salt A + Salt B + H₂O → Crystals of Addition Compound

Types of Addition Compounds

Double Salt

Completely dissociates in water
Loses identity in aq. solution
Each ion gives its test
Individual ions retain identity

e.g. Carnalite, Potash Alum, Mohr's Salt

Complex Salt

Does NOT completely dissociate
Retains identity in solution
Each ion doesn't give the test
Individual ions lose identity

e.g. K₄[Fe(CN)₆] → 4K⁺ + [Fe(CN)₆]⁴⁻

Key Distinction

[Cu(NH₃)₄]·SO₄ in water gives test of: Cu²⁺ & NH₃ ions → Complex Salt (partially dissociates)

Carnalite/Potash Alum → completely dissociates → Double Salt

Why d-Block Metals Form More Complexes

They have vacant d orbitals and small size — ideal for coordinate bond formation with ligands.

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REPRESENTATION & KEY DEFINITIONS

Representation Format

K₄ [ Fe (CN)₆ ]

Counter Ion → Ionisation Sphere | Central Metal + Ligand → Co-ordination Sphere

Rule: First sphere = Cation, Second sphere = Anion (always)

Important Definitions

1. Homoleptic Complex

Same type of ligands in complex. e.g. [Fe(CN)₆]⁴⁻, [Co(en)₃]³⁺

2. Heteroleptic Complex

More than one type of ligand. e.g. [Pt(NH₃)₂Cl₂], [CoCl₂(en)₂]⁺

3. Co-ordination Entity

Central metal atom/ion bonded to fixed number of ions or molecules (ligands), enclosed in square brackets.

4. Central Atom/Ion

Lone pair acceptor. Generally referred to as Lewis Acid.

5. Co-ordination Number (CN)

Equal to number of σ bonds between ligand & central metal ion.

Note: π-coordinate bonds are NOT included in CN count.

[Pt(Cl₄)]²⁻ → CN = 4 (Monodentate)
[Co(en)₃]Cl₃ → CN = 6 (Bidentate en)

6. Oxidation Number of Central Atom

Charge on CMI if all ligands removed along with electron pairs.

K₄[Fe(CN)₆]: +4 + x − 6 = 0 → x = +2

[Co(en)₃]Cl₃: x + 0 − 3 = 0 → x = +3

Co-ordination Polyhedron

Geometry	CN	Example
Linear	2	[Ag(NH₃)₂]⁺
Trigonal Planar	3	[HgI₃]⁻
Tetrahedral	4	[Ni(CO)₄]
Square Planar	4	[Ni(CN)₄]²⁻
Trigonal Bipyramidal	5	[Fe(CO)₅]
Octahedral	6	[Co(NH₃)₆]³⁺

Neutral Complexes

Neutral complexes are generally water insoluble. e.g. [Ni(CO)₄]°

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CLASSIFICATION OF LIGANDS

Ligand — Definition

Molecules/ions capable of donating e⁻ pairs to CMI. Lewis Bases can work as ligands.

A. On Basis of Charge

Anionic Ligands [Suffix = 'O']

SO₄²⁻ : Sulphato
CO₃²⁻ : Carbonato
Cl⁻ : Chlorido
Br⁻ : Bromido
OH⁻ : Hydroxido
F⁻ : Fluorido
CN⁻ : Cyanido
NC⁻ : Isocyanido
NO₂⁻ : Nitrito-N
ONO⁻ : Nitrito-O
SCN⁻ : Thiocyanato-S
NCS⁻ : Thiocyanato-N
C₂O₄²⁻ : Oxalato
NH₂CH₂COO⁻ : Glycinato
dmg⁻ : Dimethylglyoximato
acac⁻ : Acetylacetonato
edta⁴⁻ : Ethylenediaminetetraacetato

CN⁻, NO₂⁻, SCN⁻ are Ambidentate ligands

Neutral Ligands

H₂O : Aqua
NH₃ : Ammine
CO : Carbonyl
NO : Nitrosyl
PH₃ : Phosphine
PPh₃ : Triphenylphosphine
Py : Pyridine
en : Ethylenediamine (d=2)
pn : Propylenediamine (d=2)
tn : Trimethylenediamine (d=2)
bn : Butylenediamine (d=2)
dien : Diethyltriamine (d=3)
trien : Triethylenetetramine (d=4)
dipy : Bipyridyl (d=2)

Cationic Ligands

NO⁺ : Nitrosonium
NH₂−NH₃⁺ : Hydrazenium

B. On Basis of Denticity

Denticity

Number of e⁻ pairs which can be donated by a ligand to metal ion simultaneously.

Type	Denticity	Examples
Unidentate	d = 1	F⁻, Cl⁻, Br⁻, I⁻, NH₃, PH₃
Bidentate	d = 2	en, C₂O₄²⁻, gly⁻, dmg⁻, acac⁻
Tridentate	d = 3	dien
Tetradentate	d = 4	trien
Pentadentate	d = 5	edta³⁻ (donor: 3O + 2N)
Hexadentate	d = 6	edta⁴⁻ (donor: 4O + 2N)

C. Ambidentate & Flexidentate Ligands

Ambidentate

More than 1 donor atom but NOT simultaneously functional.

e.g. CN⁻, NO₂⁻, SCN⁻

CN⁻ → cyanide (donor C) | NC⁻ → isocyanide (donor N)

Flexidentate

Can show variable denticity.

CO₃²⁻ : d = 1 or 2
SO₄²⁻ : d = 1 or 2
edta : d = 6/5/4/3/2

D. Classical vs Non-Classical Ligands

Classical Ligand

Only σ bond by donating e⁻ pairs to CMI.

e.g. NH₃, F⁻, H₂O, OH⁻

Non-Classical (π-acid) Ligand

σ-donor + π-acceptor. Accept e⁻ density from metal by backbonding.

e.g. CO, CN⁻, NO, C₂H₄, C₆H₆, PPh₃

Type of π-bond: dπ–pπ bonding

MDJ Trick

In Ligands, the donor atom of 2nd period CANNOT form π-coordinate bonds (no vacant d orbital).

So F⁻, O²⁻ (2nd period) → only classical ligands.

Q: CN⁻ is a: (a) Monodentate (b) Ambidentate (c) π-acid ligand (d) All of these

✅ Ans: (d) All of these — CN⁻ is monodentate, ambidentate AND π-acid ligand

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IMPORTANT LIGAND STRUCTURES

Ligand	Key Properties	Ring/Donor
en (ethylenediamine)	Bidentate, Symmetrical, Neutral	5-membered ring, 2N donor
C₂O₄²⁻ (oxalato)	Bidentate, Anionic (−2), Symmetrical	5-membered ring, 2O donor
dmg⁻ (dimethylglyoximato)	Bidentate, Symmetrical	5-membered ring, 2N donor
gly⁻ (glycinato)	Bidentate, Asymmetrical (N+O)	5-membered ring, 1N+1O donor
acac⁻ (acetylacetonato)	Bidentate, Symmetrical	6-membered ring, 2O donor
edta⁴⁻	Hexadentate	5 rings (each 5-membered), 4O+2N
dien	Tridentate, Symmetrical	2×5-membered rings, 3N donor
trien	Tetradentate, Symmetrical	3×5-membered rings, 4N donor
dipy (bipyridyl)	Bidentate, Symmetrical	4 five-membered ring, 2N donor

Ni(dmg)₂ Complex — Important Points

Key Facts about Ni(dmg)₂

dmg⁻ is a didentate ligand
CN of Ni²⁺ is four
Square planar geometry, dsp² hybridisation of Ni²⁺
Neutral complex, water insoluble, rosy red ppt
Formed during test of Ni²⁺ ion
Has POS → optically inactive complex
Dimerisation b/w two dmg⁻ due to symmetric N-Bonding
Total 4 rings: Two 5-membered + Two 6-membered

EDTA⁴⁻ Complex — Key Facts

Hexadentate (d=6), Donor atom = 4O + 2N
Total rings = 5 (each is 5-membered)
Non-coplanar rings → optically active complex
Octahedral geometry of complex
[Ca(edta)]²⁻ → removes lead poisoning
Na₂-edta → estimates water hardness

Q: How many edta ligands required to form octahedral complex with Ca?

✅ Ans: (c) 1 — edta is hexadentate, one edta fills all 6 positions

π e⁻ Donor Ligands (Hapticity)

Hapticity (η)

For π-donor ligands, hapticity (η) is used instead of denticity.

ηˣ: x = no. of contiguous atoms in which e⁻ cloud is delocalized

Ligand	Hapticity	Donation
C₂H₄ (ethylene)	η²	2π e⁻ donation
C₄H₄ (cyclobutadiene)	η⁴	4π e⁻ donation
C₆H₆ (benzene)	η⁶	6π e⁻ donation
C₅H₅⁻ (cyclopentadienyl) σ	η¹	2e⁻ donation (σ)
C₅H₅⁻ (cyclopentadienyl) π	η⁵	6e⁻ donation (π)
C₃H₅⁻ (allyl) σ	η¹	2e⁻ donation
C₃H₅⁻ (allyl) π	η³	4e⁻ donation

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EAN & SIDGWICK RULE

EAN = Atomic no. of metal (Z) − O.N. of Metal + e⁻ gained by Ligands

MDJ Trick

Metal ke paas e⁻ ka hisab kitab: [Kitne Gaye] (+ charge / O.N.) minus → [Kitne Aaye] (from Ligands / e⁻ gain)

EAN Examples

Complex	Z	O.N.	e⁻ from Ligand	EAN
K₄[Fe(CN)₆]	26	+2	12 (6×2)	36
[Ni(en)₃]²⁺	28	+2	12 (6×2)	38
[Fe(CO)₅]	26	0	10 (5×2)	36
Ni(CO)₄	28	0	8 (4×2)	36
[Fe(π-C₅H₅)₂] Ferrocene	26	+2	12 (6×2)	36
[Cr(C₆H₆)₂] Chromocene	24	0	12 (6×2)	36
[Cr(CO)₆]	24	0	12 (6×2)	36

Sidgwick Rule of EAN

Complex will be stable if its EAN = Atomic no. of next inert gas (36, 54, 86...).

Note: This rule fails in many cases but works best for stability of Metal Carbonyls (metals in lower OS: 0/−1).

Special EAN Cases

Dimerisation to Achieve EAN = 36

Mn(CO)₅: EAN = 25−0+10 = 35 (unstable)

2 Mn(CO)₅ → Mn₂(CO)₁₀ → EAN per Mn = 36 ✓

[Mn(CO)₅]: Works as reducing agent (loses e⁻ to become [Mn(CO)₅]⁺ with EAN=36)

V(CO)₆: Works as oxidising agent (gains e⁻ → [V(CO)₆]⁻ with EAN=36)

NO as Electron Donor

NO acts as 3e⁻ donor | NO⁺ acts as 2e⁻ donor

Synergic Bonding / Back Bonding (CO Ligand)

CO is a π-acid ligand. Two types of bond formed:

① σ-bond: C donates lone pair to metal (σ-donor)

② π back-bond: Metal donates filled d e⁻ to vacant π* of CO (π-acceptor)

Order of back bonding: [M(CO)ₓ]⁻ > [M(CO)ₓ]⁰ > [M(CO)ₓ]⁺

More −ve charge on M → more back donation tendency ↑

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IUPAC NAMING OF COMPLEXES

IUPAC Naming Rule

Name of Counter Ion (Cation) → [No. of Ligands + Name of Ligands (alphabetically) + Name of Metal + O.N. in Roman No.] → Name of Counter Ion (Anion)

Rules for Ligand Names

Anionic ligands → Suffix "o"
Organic anionic ligand → Suffix "yl" (e.g. C₅H₅⁻)
If complex is anionic → use "ate" suffix with metal name

Metal Names in Anionic Complexes

Fe → Ferrate | Sn → Stannate | Pb → Plumbate
Ag → Argentate | Au → Aurate | Cu → Cuprate

Co → Cobaltate | Zn → Zincate | Ni → Nickelate
Cr → Chromate | Pt → Platinate | Pd → Palladate

Numerical Prefixes

Simple ligand: mono, di, tri, tetra, penta, hexa

Complex ligand name (has numerical prefix): bis, tris, tetrakis — place ligand in parentheses

IUPAC Name Examples

Formula	IUPAC Name
K₄[Fe(CN)₆]	Potassium hexacyanidoferrate(II)
[Ni(CO)₄]	Tetracarbonyl nickel(0)
[CoCl₂(en)₂]⁺	Dichloro bis(ethylenediamine) cobalt(III)
[Co(NH₃)₄(H₂O)Cl]Cl₂	Tetraammine aquachlorido cobalt(III) chloride
[Pt(NH₃)₂Cl(NO₂)]	Diammine chloro nitrito-N platinum(II)
K₃[Al(C₂O₄)₃]	Potassium trioxalato aluminate(III)
K₂[PdCl₄]	Potassium tetrachlorido palladate(II)
[Fe(π-C₅H₅)₂]	Bis(η⁵-cyclopentadienyl) iron(II)
K[Pt Cl₂(C₂H₄)] Zeise's salt	Potassium trichloriodo(η²-ethylene)platinate(II)
[Cr(C₆H₆)₂] Chromocene	Bis(η⁶-benzene) chromium(0)

Special: Brown Ring Complex

Formed during test of NO₃⁻/NO₂⁻ ions.

Formula: [Fe(H₂O)₅·NO]SO₄ | Name: Pentaaqua nitrosonium ferrate(I) sulphate

When Fe & NO together → NO becomes NO⁺

Prussian Blue

Iron(III) hexacyanidoferrate(II): Fe³⁺ + [Fe(CN)₆]⁴⁻ → Fe₄[Fe(CN)₆]₃

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WERNER'S THEORY

According to Werner, metal ion in a complex has TWO types of valency:

Primary Valency (1°)

Equal to O.N. of metal
Satisfied by anions (ionisable)
Cannot predict geometry
Non-directional
Represented by dashes (---)

Secondary Valency (2°)

Equal to C.N. of metal
Satisfied by anions + neutral molecules
Can predict geometry
Non-ionisable, Directional
Represented by lines (—)

Werner Diagram Examples

PtCl₄·4NH₃ → [Pt(NH₃)₄Cl₂]Cl₂

1° (O.N.) = 4 | 2° (C.N.) = 6
No. of ionisable Cl = 2 → 50% ionisable
Mol of AgCl ppt = 2 | Electrolyte type = 1:2

Werner's Observations — CrCl₃·nNH₃

Compound	AgCl ppt	Formula	No. of ions	Electrolyte
CrCl₃·6NH₃ (A)	3 AgCl↓	[Cr(NH₃)₆]Cl₃	4	1:3
CrCl₃·5NH₃ (B)	2 AgCl↓	[Cr(NH₃)₅Cl]Cl₂	3	1:2
CrCl₃·4NH₃ (C)	1 AgCl↓	[Cr(NH₃)₄Cl₂]Cl	2	1:1

Key Rule

No. of AgCl ppt per mol = No. of ionisable Cl⁻ ions (outside the box / coordination sphere).

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VALENCE BOND THEORY (VBT)

C.N.	Geometry	Hybridisation	d orbital used
2	Linear	sp	—
3	Trigonal Planar	sp²	—
4	Tetrahedral	sp³	—
4	Square Planar	dsp²	dx²−y²
5	Trigonal Bipyramidal	dsp³ / sp³d	dz²
6	Octahedral	d²sp³ / sp³d²	dz², dx²−y²

Strong vs Weak Ligand (MDJ)

Strong Ligands (C,N donor): Can pair d electrons → inner 'd' orbital used → low spin, diamagnetic

Weak Ligands (O, halogen donor): Cannot pair d electrons → outer 'd' orbital used → high spin, paramagnetic

Donation strength: C > N > O > X

MDJ — d⁻ Electron Count Trick

No. of 'd' e⁻ in M²⁺ state of all d-block elements = their serial number (1 to 10)

e.g. Sc²⁺=d¹, Ti²⁺=d², ... Zn²⁺=d¹⁰

Important VBT Examples

K₄[Fe(CN)₆]: Fe²⁺ = d⁶, CN⁻ = Strong Ligand

d²sp³ hybridisation → Octahedral → n=0 → Diamagnetic, Low Spin, Inner 'd' complex

K₃[FeF₆]: Fe³⁺ = d⁵, F⁻ = Weak Ligand

sp³d² hybridisation → Octahedral → n=5 → Paramagnetic, High Spin, Outer 'd' complex

Ni(CO)₄: Ni⁰ = 3d¹⁰ (after promotion 3d¹⁰4s⁰)

CO = Strong Ligand → sp³ → Tetrahedral, Diamagnetic, Low Spin

[Ni(CN)₄]²⁻: Ni²⁺ = d⁸, CN⁻ = Strong Ligand

dsp² hybridisation → Square Planar, Diamagnetic

[NiCl₄]²⁻: Ni²⁺ = d⁸, Cl⁻ = Weak Ligand

sp³ → Tetrahedral, n=2 Paramagnetic

MDJ Shortcuts (Important!)

d¹/d²/d³ with CN=6 → Always d²sp³ (inner d, para)
d⁸/d⁹/d¹⁰ with CN=6 → Always sp³d² (outer d)
d¹⁰ with CN=4 → Always sp³ (Tetra)
d¹⁰ with CN=6 → Always sp³d² (Octa)
4d/5d series metals → Strong Ligand with any ligand
M⁴⁺ → All ligands will be S.L.
Cr²⁺/Mn²⁺/Fe²⁺/Co²⁺ + NH₃ → WL

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CRYSTAL FIELD THEORY (CFT)

Basis of CFT

Based on electrostatic model. Metal cation = point +ve charge. Ligand = point −ve charge (neutral ligand = point dipole).

When ligand approaches → energy of d orbital increases (e⁻−e⁻ repulsion) → d orbital splitting occurs.

Splitting in Octahedral Field

Octahedral Splitting (Δₒ)

Ligands attack along the axis → axial orbitals (dz², dx²−y²) feel MORE repulsion

eg orbitals: +0.6 Δₒ (dz², dx²−y²)

t₂g orbitals: −0.4 Δₒ (dxy, dyz, dzx)

Splitting in Tetrahedral Field

Tetrahedral Splitting (Δt)

Ligands attack between axes (non-axial) → non-axial orbitals (dxy, dyz, dzx) feel MORE repulsion

t₂ orbitals: +0.4 Δt (dxy, dyz, dzx)

e orbitals: −0.6 Δt (dz², dx²−y²)

NOTE: In tetrahedral field, Δt < P (always) → Always High Spin (WL)

Δt = (4/9) Δₒ | Δsp = 1.3 Δₒ | Always: Δt < Δₒ < Δsp

Electronic Configuration in Octahedral Field

MDJ Filling Order

Strong Ligand: t₂g → t₂g → eg → eg

Weak Ligand: t₂g → eg → t₂g → eg

d-config	Strong Ligand (SL)	Weak Ligand (WL)
d¹,d²,d³	t₂g¹⁻³ eg⁰	t₂g¹⁻³ eg⁰ (same)
d⁴	t₂g⁴ eg⁰	t₂g³ eg¹
d⁵	t₂g⁵ eg⁰	t₂g³ eg²
d⁶	t₂g⁶ eg⁰	t₂g⁴ eg²
d⁷	t₂g⁶ eg¹	t₂g⁵ eg²
d⁸,d⁹,d¹⁰	Same for SL and WL

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SPECTROCHEMICAL SERIES & CFSE

Spectrochemical Series (Increasing Field Strength)

I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < edta⁴⁻ < NH₃ < Py < en < dipy < NO₂⁻ < CN⁻ < CO

Stability of Complexes

Stability ∝ Δ (Splitting Energy)

5 Factors affecting Δ (CFSE):

Nature of Ligand (spectrochemical series)
Charge on CMI (higher charge → higher Δ)
Zeff of CMI (3d < 4d < 5d series)
Geometry (Δsp > Δₒ > Δt)
Chelation (most dominating factor)

Chelation Examples

[Ni(H₂O)₆]²⁺ < [Ni(en)₃]²⁺
[Co(H₂O)₆]³⁺ < [Co(en)₃]³⁺
[Co(en)₃]³⁺ > [Co(NO₂)₆]³⁻ (en is chelate)
[Co(en)₃]³⁺ < [Co(edta)]⁻² (5 rings vs 3 rings)

CFSE Calculation (Octahedral)

CFSE = n(−0.4Δₒ) + m(+0.6Δₒ) + xP

Where: n = no. of e⁻ in t₂g, m = no. of e⁻ in eg, P = pairing energy, x = no. of new pairs formed

Zero CFSE Systems

d⁵ (WL): t₂g³eg² → CFSE = 0 (also called High Spin d⁵)

d¹⁰ (SL/WL): t₂g⁶eg⁴ → CFSE = 0 (natural pairing)

Q: If Δₒ of [CoCl₆]⁴⁻ is 18000 cm⁻¹, find Δt for [CoCl₄]²⁻?

✅ Ans: Δt = (4/9) × 18000 = 8000 cm⁻¹

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COLOUR IN COMPLEXES

Why Colour?

Complexes are coloured due to d-d transition explained by CFT on basis of splitting.

Energy absorbed = Splitting energy Δ. E ∝ 1/λ → Colour of complex is complementary to absorbed colour

d⁰ & d¹⁰ → Colourless

No d-d transition possible in aqueous medium.

But may be coloured due to: LMCT, MLCT, MMCT

d¹ to d⁹ → Coloured

d-d transition possible.

Coloured d⁰ Examples

KMnO₄ (Mn = d⁰): Purple due to LMCT

K₂Cr₂O₇ (Cr = d⁰): Orange due to LMCT

Fe₄[Fe(CN)₆]₃ (Prussian Blue): Colour due to MMCT

Possible Reasons for Colour

d-d transition
HOMO-LUMO transition (responsible for colour in halogens)
MMCT, LMCT, MLCT (Charge transfer)
Polarisation

Complementary Colour Chart (VIBGYOR)

VI → B → G → Y → O → R
λ increases (400–700nm) | E decreases

Strong ligand → High Δ → absorbs high energy (left side) → shows complementary colour (right side)

MDJ: Strong Ligand → High Energy Colour Absorb Karta Hai

M(en)₃ > M(H₂O)₆ > M(Cl)₆ → Δ order A > B > C → λ absorbed A < B < C

Colour Change with Ligand Substitution (Ni example)

[Ni(H₂O)₆]²⁺ → Green | + en → [Ni(H₂O)₄(en)]²⁺ Pale Blue

[Ni(H₂O)₂(en)₂]²⁺ → Blue | [Ni(en)₃]²⁺ → Violet

Gem Stones Colour

Ruby (red) and Emerald (green) colour due to Cr³⁺ (d³) ions — d-d transition

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ISOMERISM IN CO-ORDINATION COMPOUNDS

Structural Isomerism

Ionisation Isomerism
Linkage Isomerism
Hydrate Isomerism
Co-ordination Isomerism

Stereo Isomerism

Geometrical Isomerism (G.I.)
Optical Isomerism (O.I.)

1. Ionisation Isomerism

Isomers which produce different ions on ionisation. Counter ion in salt is itself a potential ligand.

e.g. [Pt(NH₃)₃Cl]Br ↔ [Pt(NH₃)₃Br]Cl

2. Linkage Isomerism

Due to presence of ambidentate ligands.

No. of LI = 2ⁿ where n = no. of ambidentate ligands

e.g. [Pt(NH₃)₅NO₂]Cl₂ ↔ [Pt(NH₃)₅ONO]Cl₂

3. Hydrate (Solvate) Isomerism

Occurs due to different no. of H₂O in co-ordination sphere. CoCl₃·6H₂O has 3 hydrate isomers:

[Co(H₂O)₆]Cl₃ (violet)
[Co(H₂O)₅Cl]Cl₂·H₂O
[Co(H₂O)₄Cl₂]Cl·2H₂O

4. Co-ordination Isomerism

Due to exchange of ligands b/w cationic and anionic complexes.

MDJ: Applicable for Monodentate ligands only.

e.g. [Pt(NH₃)₄][PtCl₄] ↔ [Pt(NH₃)₃Cl][PtCl₃(NH₃)]

5. Geometrical Isomerism

Not shown by homoleptic complexes. Cis = two same ligands adjacent. Trans = two same ligands opposite.

Not given by Tetrahedral geometry (but given by square planar)
Octahedral can give both G.I. and O.I.

G.I. in Square Planar (CN=4)

Type	No. of G.I.	Note
Ma₄	0
Ma₃b	0
Ma₂b₂	2	cis & trans
Ma₂bc	2
Mabcd	3
M(AA)₂	0	Sym. bidentate → always 0 G.I.
M(AB)₂	2	e.g. [Pt(gly)₂]

G.I. in Octahedral (CN=6)

Type	G.I.	O.A. Pairs	S.I. Total
Ma₆	0	0	0
Ma₅b	0	0	0
Ma₄b₂	2	0	2
Ma₃b₃	2 (fac & mer)	0	2
Ma₃bcd	4	1	5
M(AA)₃	0	2	2
M(AA)₂b₂	2	1	3

Optical Isomerism Rules (Octahedral)

If two same ligands are at any trans position → POS → Optically Inactive
If any two trans positions are mutually same (bb, bb) → POS → Optically Inactive
If Non-coplanar rings are present → Optically Active

Q: [Co(NH₃)₄(NO₂)₂]Cl exhibits which isomerisms?

✅ Ans: Linkage Isomerism + Ionisation Isomerism + Geometrical Isomerism

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ORGANOMETALLIC COMPOUNDS

Definition

When carbon is directly attached with metal: M−C bond.

σ-Bonded

RMgX (Grignard Reagent)
R₂Zn (Frankland Reagent)
R₂CuLi (Gilman Reagent)
(C₂H₅)₄Pb (Tetraethyl Lead)
TiCl₄+R₃Al / TiCl₃+R₂AlCl → Ziegler-Natta Catalyst

π-Bonded

Ferrocene [Fe(π-C₅H₅)₂]
Chromocene [Cr(C₆H₆)₂]
Ruthenocene
Zeise's Salt K[Pt Cl₃(C₂H₄)]

σ+π Bonded (Metal Carbonyls)

Fe(CO)₅, Ni(CO)₄
Cr(CO)₆, Mn₂(CO)₁₀
Co₂(CO)₈

Jahn-Teller Effect

Distortion in geometry of octahedral complexes due to asymmetrical e⁻ configuration.

Strong JT effect: Asymmetrical eg configuration (d⁹, high-spin d⁷, d⁴)

Weak JT effect: Asymmetrical t₂g configuration

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IMPORTANT COMPLEXES & APPLICATIONS

Complex	Key Facts / Application
K₄[Fe(CN)₆]	Lab reagent. Tests Fe³⁺(→ Prussian Blue), Cu²⁺(→ chocolate brown ppt), Zn²⁺(→ blue-white ppt)
K₃[Fe(CN)₆]	Lab reagent. Tests Fe²⁺ → Turnbull's Blue (ferro ferdi cyanide)
Ni(CO)₄	Purification of Ni (Mond's Process). Volatile complex. sp³ tetrahedral, diamagnetic
[Ca(edta)]²⁻	Removes lead poisoning
Na₂-edta	Estimates water hardness
[Pt(NH₃)₂Cl₂] cis	Cisplatin — anticancer drug
[Cu(NH₃)₄]SO₄	Deep blue colour complex
Na₃[Ag(S₂O₃)₂]	Photography fixing agent (dissolves undecomposed AgBr)
Na[Ag(CN)₂]	Extraction of Ag (cyanide process)
Na[Au(CN)₂]	Extraction of Au (cyanide process)
K₂[HgI₄]	Nessler's reagent. Tests NH₄⁺ ion (brown ppt)
Na₂[Fe(CN)₅NO]	Sodium nitroprusside (SNP). Tests S²⁻ → purple/violet colour (STNP)
[Rh Cl(PPh₃)₃]	Wilkon's catalyst — hydrogenation of unsaturated hydrocarbons
Cyanocobalamine	Vitamin B₁₂ → contains Co

K₃[Cu(CN)₄] vs K₂[Cd(CN)₄]

Cu²⁺ + excess KCN → Cu⁺ (reduced by CN⁻) → K₃[Cu(CN)₄] → stable, no ppt with H₂S

Cd²⁺ + excess KCN → K₂[Cd(CN)₄] → unstable, gives CdS↓ (yellow) with H₂S

Stability: K₃[Cu(CN)₄] > K₂[Cd(CN)₄]

COORDINATION COMPOUNDS