Coordination Compounds | IISER Smart Prep

1. Core Concept & Definitions

Coordination compounds consist of a central metal atom/ion bonded to a set of molecules or ions (ligands) via coordinate covalent bonds.

Important Terms:

Ligands: Ions or molecules that donate a pair of electrons.
- Unidentate: Donates one pair (e.g., Cl^-, NH₃, H₂O).
- Didentate: Donates two pairs (e.g., ethane-1,2-diamine 'en', oxalate 'ox'^2-).
- Ambidentate: Can ligate through two different atoms (e.g., NO₂^-/ONO^-, SCN^-/NCS^-).
Denticity: The number of ligating atoms in a single ligand.
Chelate Effect: When a di- or polydentate ligand uses two or more donor atoms to bind to a single metal ion, it forms a ring. This results in extra stability called the chelate effect.

EAN Rule (Effective Atomic Number)

EAN = Z - (Oxidation State) + 2(Coordination Number). If EAN equals the atomic number of the next noble gas, the complex is generally stable.

2. Werner’s Theory

Alfred Werner proposed two types of valencies for metal ions in coordination compounds:

Primary Valency: Ionizable, corresponds to the Oxidation State.
Secondary Valency: Non-ionizable, fixed, corresponds to the Coordination Number.

Experimental Evidence:

Using AgNO₃, we can determine the number of ionizable Cl^- ions.
CoCl₃ · 6NH₃ gives 3 moles of AgCl → [Co(NH₃)₆]Cl₃ (Secondary Valency = 6).
CoCl₃ · 5NH₃ gives 2 moles of AgCl → [Co(NH₃)₅Cl]Cl₂ (Secondary Valency = 6).

3. IUPAC Nomenclature

Order: Ligands (alphabetical) → Metal Name → Oxidation state (Roman numerals).
Anionic Ligands: End in '-o' (Chloro, Cyano, Nitrito-N, Nitrito-O).
Neutral Ligands: Special names (H₂O: Aqua, NH₃: Ammine, CO: Carbonyl, NO: Nitrosyl).
Metal Name: If the complex is an anion, use '-ate' (e.g., Ferrate, Platinate).

Prefixes: Use 'bis', 'tris' for ligands that already contain numerical prefixes (e.g., ethylenediamine).

4. Isomerism

Structural Isomerism:

Ionization: [Co(NH₃)₅SO₄]Br (gives AgBr) vs [Co(NH₃)₅Br]SO₄ (gives BaSO₄).
Linkage: Occurs with ambidentate ligands (M-NO₂ vs M-ONO).
Hydrate: [Cr(H₂O)₆]Cl₃ (violet) vs [Cr(H₂O)₅Cl]Cl₂ · H₂O (green).

Stereoisomerism:

Geometrical Isomerism:

Square Planar: [MA₂B₂] (cis/trans). [MABCD] has 3 isomers.
Octahedral: [MA₄B₂] type (cis/trans). [MA₃B₃] type (facial/fac and meridional/mer).

Optical Isomerism: Non-superimposable mirror images. Dextro (d) and Laevo (l) forms. Common in [M(en)₃]ⁿ⁺ and [M(en)₂Cl₂]⁺.

5. Bonding Theories

Valence Bond Theory (VBT):

CN	Hybridisation	Geometry	Orbitals Used
4	sp³	Tetrahedral	s, p_x, p_y, p_z
4	dsp²	Square Planar	d_x²-y², s, p_x, p_y
6	d²sp³	Octahedral (Inner)	d_z², d_x²-y², s, p_x, p_y, p_z

Crystal Field Theory (CFT):

Octahedral Field: Ligands approach along the axes. Orbitals on axes (d_x²-y², d_z² - e_g set) experience more repulsion than orbitals between axes (d_xy, d_yz, d_zx - t_2g set).

CFSE = [-0.4n(t_2g) + 0.6n(e_g)] Δ_o + mP

n = number of electrons, P = pairing energy.

Spectrochemical Series: I^- < Br^- < S^2- < SCN^- < Cl^- < S^2- < F^- < OH^- < C₂O₄^2- < H₂O < NCS^- < edta^4- < NH₃ < en < CN^- < CO

6. Bonding in Metal Carbonyls

Metal carbonyls involve Synergic Bonding:

σ-Bond: Donation of lone pair from CO carbon into a vacant metal orbital.
π-Bond: Back-donation of electrons from a filled metal d-orbital into the vacant anti-bonding π* orbital of CO.

Result: Back-bonding increases the M-C bond strength but decreases the C-O bond strength (increases C-O bond length).

7. Importance & Applications

Qualitative Analysis: Ni²⁺ identified using DMG (forms red complex).
Metallurgy: Extraction of Silver/Gold via cyanide process.
Biology: Chlorophyll (Mg), Hemoglobin (Fe), Vitamin B12 (Co).
Medicine: Cis-platin used in cancer treatment. EDTA used for lead poisoning treatment.
Catalysis: Wilkinson catalyst [RhCl(PPh₃)₃] for hydrogenation.

8. Common Mistakes

Ambidentate Check: Always check if NO₂ is bonded via N or O (Nitrito-N vs Nitrito-O).
Low Spin vs High Spin: For d⁴ to d⁷ octahedral ions, always compare Δ_o and P before filling electrons.
Geometric Isomers: Square planar [MA₃B] or [MAB₃] types have zero geometrical isomers.

9. IAT Exam Focus Points

Color: Complementary colors (e.g., if yellow light is absorbed, the compound appears violet).
Optical Isomerism: Only cis monomers of [M(AA)₂B₂] type are optically active; trans is inactive (has plane of symmetry).
Number of Ions: Calculation based on Kohlrausch law or precipitation data.

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