1. Basic Terminology

Coordination Compounds: Compounds containing a central metal atom bonded to a fixed number of ions or molecules (ligands) by coordinate bonds. Example: K₄[Fe(CN)₆].

Double Salt vs Complex:

Double Salt: Dissociates completely into ions in water (e.g., Mohrs salt, Potash Alum).
Complex: Retains identity in solid and solution state. The complex ion [Fe(CN)₆]^4- does not dissociate into Fe²⁺ and CN^-.

Key Terms

Coordination Entity: The central metal + ligands enclosed in square brackets [ ].
Central Metal: Lewis Acid (accepts electron pairs). Usually transition metals.
Ligands: Lewis Bases (donate electron pairs). Can be neutral (NH₃), negative (Cl^-), or positive (NO⁺).
Coordination Number (CN): Number of coordinate bonds formed with the central metal. (For monodentate ligands, CN = Number of ligands).

2. Ligands & Nomenclature

Types of Ligands

Type	Description	Examples
Unidentate	Donates 1 pair.	H₂O (aqua), NH₃ (ammine), Cl^- (chlorido)
Bidentate	Donates 2 pairs.	en (ethane-1,2-diamine), ox (oxalate)
Polydentate	Multiple pairs.	EDTA^4- (Hexadentate)
Ambidentate	Two donor atoms but uses one.	NO₂^- (N-nitrito), ONO^- (O-nitrito)

IUPAC Nomenclature Rules:

Cation is named first, then anion.
Ligands are named alphabetically before the metal.
Prefixes (di, tri, tetra) used. For complex ligands (bis, tris, tetrakis).
Oxidation state of metal in Roman numerals ( ).
If complex is anionic, metal ends in -ate (e.g., Ferrate, Argentate).

3. Werner's Theory

Alfred Werner proposed that metals show two types of valencies:

Primary Valency: Ionizable. Equals oxidation state. Satisfied by anions. (Represented by dotted lines).
Secondary Valency: Non-ionizable. Equals coordination number. Satisfied by ligands. (Represented by solid lines). Directional (gives geometry).

4. Isomerism

Structural Isomerism

Linkage: Ambidentate ligands (NO₂ vs ONO).
Coordination: Exchange of ligands between cationic and anionic entities.
Ionization: Different ions in solution (Counter ion becomes ligand). e.g., [Co(NH₃)₅SO₄]Br vs [Co(NH₃)₅Br]SO₄.
Solvate (Hydrate): Water as ligand vs water of crystallization.

Stereoisomerism

Geometrical (Cis-Trans):

Common in CN=4 (Square Planar) and CN=6 (Octahedral).
Fac-Mer Isomerism: In Octahedral [MA₃B₃]. Fac (facial) and Mer (meridional).
Note: Tetrahedral complexes do NOT show geometrical isomerism.

Optical (Enantiomers):

Non-superimposable mirror images. Chiral. Common in octahedral complexes with bidentate ligands (e.g., [Co(en)₃]³⁺).

5. Bonding (VBT & CFT)

Valence Bond Theory (VBT)

Hybridization determines geometry.

CN=4	sp³ (Tetrahedral)	dsp² (Square Planar)
CN=6	sp³d² (Outer Orbital)	d²sp³ (Inner Orbital)

Crystal Field Theory (CFT)

Considers ionic interaction. Ligands split metal d-orbitals.

Octahedral Splitting (Δ_o):

Split into t_2g (lower energy, d_xy, d_yz, d_zx) and e_g (higher energy, d_x2-y2, d_z2).

Strong Field Ligand: High Δ_o. Causes pairing. (Low Spin).
Weak Field Ligand: Low Δ_o. No pairing. (High Spin).

[Image of Crystal Field Splitting in Octahedral]

Spectrochemical Series:
I^- < Br^- < Cl^- < F^- < OH^- < H₂O < NH₃ < en < CN^- < CO
(Weak Field → Strong Field)

6. Colour & Applications

Colour:

Due to d-d transition. Electron absorbs light to jump from t_2g to e_g. The complementary colour is observed.
Anhydrous CuSO₄ is white (no ligands, no splitting). CuSO₄·5H₂O is blue.

Applications:

Hardness of Water: Estimation using EDTA.
Medicine: Cis-platin (Cancer treatment).
Extraction: Silver and Gold (Cyanide process).
Biology: Hemoglobin (Fe), Chlorophyll (Mg), Vitamin B₁₂ (Co).

Important Formulae

1. Spin Only Magnetic Moment

μ = √[n(n + 2)] B.M.

n = Number of unpaired electrons

2. Effective Atomic Number (EAN)

EAN = Z - O.S. + 2 × C.N.

Z = Atomic Number
O.S. = Oxidation State
C.N. = Coordination Number

(Stability Rule: If EAN = Noble Gas Z, complex is stable - Sidgwick Rule)

3. CFSE (Octahedral)

CFSE = [-0.4(n_t2g) + 0.6(n_eg)] Δ_o + nP

n_t2g = Electrons in t_2g
n_eg = Electrons in e_g
P = Pairing Energy

Relation Δ_t and Δ_o:

Δ_t = (4/9) Δ_o

20 Golden Facts (NEET)

1. Synergic Bonding: In Metal Carbonyls (e.g., [Ni(CO)₄]), the M-C bond possesses both σ and π character. M ← C (σ bond) and M → C (π back-bond). This strengthens the M-C bond and weakens the C-O bond.
2. Chelate Effect: Complexes with chelating ligands (like en, EDTA) are significantly more stable than those with monodentate ligands. This is an entropy-driven process.
3. Tetrahedral vs Square Planar: For Coordination Number 4:
Weak Field Ligand → Tetrahedral (sp³).
Strong Field Ligand → Square Planar (dsp²).
Exception: [Ni(CO)₄] is tetrahedral despite Strong Field CO (d¹⁰ case).
4. Hybridization of [Cu(NH₃)₄]²⁺: It is dsp² (Square Planar). One 3d electron is promoted to 4p orbital to make d-orbital available.
5. Optical Isomerism: Square planar complexes usually do NOT show optical isomerism because they possess a plane of symmetry. Tetrahedral complexes with unsymmetrical bidentate ligands can show it.
6. Zero Oxidation State: Transition metals in metal carbonyls (e.g., Ni(CO)₄, Fe(CO)₅) have zero oxidation state.
7. Colour Intensity: Tetrahedral complexes are generally more intensely coloured than octahedral ones because "d-p mixing" is allowed (lack of center of symmetry), relaxing the selection rules.
8. Spectrochemical Series: I^- < Br^- < Cl^- < F^- < OH^- < H₂O < NH₃ < en < CN^- < CO.
(Halogens are weak, Carbon/Nitrogen donors are strong).
9. Stability Constant (K): Higher value of log K means higher stability. Stability decreases with increase in ionic radius of central metal.
10. Linkage Isomerism: Shown by ambidentate ligands like NO₂ (binds through N) and ONO (binds through O), or SCN and NCS.
11. Inner vs Outer Orbital:
d²sp³ = Inner Orbital = Low Spin (Strong Ligand).
sp³d² = Outer Orbital = High Spin (Weak Ligand).
12. Brown Ring Complex: Formula is [Fe(H₂O)₅(NO)]²⁺. Here Iron is in +1 Oxidation State (rare) and NO is NO⁺.
13. Wilkinson Catalyst: [(Ph₃P)₃RhCl]. Used for hydrogenation of alkenes.
14. Denticity of EDTA: It is a hexadentate ligand with 4 Oxygen and 2 Nitrogen donor atoms. Used to treat Lead poisoning.
15. Unpaired Electrons:
Strong field ligands force pairing (reduce n).
Weak field ligands do not force pairing (max n).
16. Splitting Energy Order: Δ_sp (Square Planar) > Δ_o (Octahedral) > Δ_t (Tetrahedral).
17. Facial & Meridional: Stereoisomers of [MA₃B₃] type.
Facial (fac): Ligands on same face.
Meridional (mer): Ligands on the meridian.
18. Zeise's Salt: K[PtCl₃(η²-C₂H₄)]. First organometallic compound synthesized. Ethene is the ligand.
19. Magnetic Moment of Fe³⁺:
Weak Ligand (H₂O): d⁵ (5 unpaired), μ = 5.9 B.M.
Strong Ligand (CN^-): d⁵ (1 unpaired), μ = 1.73 B.M.
20. Cis-Platin: [Pt(NH₃)₂Cl₂] (cis-isomer). Used as an anti-cancer drug. The trans-isomer is toxic and inactive.

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