Board exam focus — Coordination Compounds (CBSE & HBSE)

Werner's theory, IUPAC nomenclature, isomerism (structural and stereoisomerism), valence bond theory (VBT), crystal field theory (CFT) and effective atomic number (EAN).

Werner's Theory and Nomenclature

Coordination Compounds

Introduction

Coordination compounds (complex compounds) contain a central metal atom/ion surrounded by coordinating ligands. Example: [Co(NH3)6]Cl3 - cobalt hexamminechloride

Werner's Theory (1893)

1. Metals exhibit PRIMARY and SECONDARY valences.

• PRIMARY valence = oxidation state of metal (ionizable; satisfied by anions)
• SECONDARY valence = coordination number of metal (non-ionizable; satisfied by neutral or anionic ligands)

1. Secondary valence directed toward fixed positions in space → geometry.

Example: [CoCl3(NH3)6] - Werner Co: primary valence = 3 (Co3+); secondary valence = 6 (CN = 6) 3 Cl- satisfy primary valence (ionizable) 3 NH3 (and possibly 3 Cl-) satisfy secondary valence (non-ionizable)

Key Definitions

Central atom: Metal ion at center of coordination sphere. Ligands: Atoms/molecules/ions that coordinate to central metal. Coordination sphere: Central atom + ligands (enclosed in square brackets). Coordination number (CN): Number of ligand atoms directly bonded to metal.

Types of Ligands:

• Monodentate: One donor atom (NH3, Cl-, H2O, CN-, CO)
• Bidentate: Two donor atoms (en = ethylenediamine, ox = oxalate C2O4^2-)
• Polydentate: Many donor atoms (EDTA: 6 donor atoms, hexadentate)

Chelate: Complex with polydentate ligand forming rings. More stable than non-chelate (chelate effect).

IUPAC Nomenclature

Rules:

1. Cation named before anion
2. Ligands named before metal, in alphabetical order
3. Anionic ligands end in -o (Cl- = chlorido, CN- = cyanido, OH- = hydroxido)
4. Neutral ligands use name (aqua=H2O, ammine=NH3, carbonyl=CO)
5. Prefix for multiple same ligands: di-, tri-, tetrakis-, pentakis-
6. Metal oxidation state in parentheses (Roman numerals)
7. Anionic complex: metal name ends in -ate (ferrate, platinate, cuprate, nickelate)

Examples: [Co(NH3)6]Cl3 = hexaamminecobalt(III) chloride [CrCl2(NH3)4]+ = tetraamminedichloridochromium(III) ion K2[PtCl4] = potassium tetrachloridoplatinate(II) [Fe(CN)6]^3- = hexacyanidoferrate(III) ion K3[Fe(CN)6] = potassium hexacyanidoferrate(III)

Effective Atomic Number (EAN)

EAN = atomic number of metal - electrons lost + electrons gained from ligands For stable complexes, EAN = atomic number of nearest noble gas.

[Co(NH3)6]^3+: Co(+3) = 27 - 3 = 24 core electrons + 12 electrons from 6 NH3 = 36 = Kr (stable)

Isomerism in Coordination Compounds

Isomerism in Coordination Compounds

Structural Isomerism

1. Ionization Isomerism: Differ in ions inside and outside coordination sphere. Example: [Co(NH3)5Br]SO4 (liberates SO4^2- in solution, gives white ppt with Ba2+) [Co(NH3)5SO4]Br (liberates Br- in solution, gives pale yellow ppt with AgNO3)

2. Hydrate (Solvate) Isomerism: Water may be inside or outside coordination sphere. Example: [Cr(H2O)6]Cl3 (violet, all 6 H2O in sphere) [Cr(H2O)5Cl]Cl2.H2O (blue-green, 5 H2O in sphere) [Cr(H2O)4Cl2]Cl.2H2O (dark green, 4 H2O in sphere)

3. Linkage Isomerism: Ambidentate ligands can coordinate through different atoms. Example: NO2- can coordinate through N (nitro, -NO2) or O (nitrito, -ONO) [Co(NH3)5NO2]^2+: N-bonded (nitro) = yellow [Co(NH3)5ONO]^2+: O-bonded (nitrito) = red

4. Coordination Isomerism: In salts with complex cation and complex anion; ligands exchange between them. Example: [Co(NH3)6][Cr(CN)6] ↔ [Cr(NH3)6][Co(CN)6]

Stereoisomerism

1. Geometric (cis-trans) Isomerism: Due to different arrangements of ligands around metal.

Square planar [MA2B2] type: cis: Two A ligands on same side trans: Two A ligands on opposite sides Example: Pt(NH3)2Cl2 cis-platin: cis isomer (anticancer drug, used in chemotherapy) trans-platin: trans isomer (no anticancer activity)

Octahedral [MA2B4] type (mer/fac): fac (facial): Three A ligands on one face of octahedron mer (meridional): Three A ligands along meridian

2. Optical Isomerism: Non-superimposable mirror images (enantiomers); rotate plane-polarized light. Occurs when complex has no plane of symmetry.

cis-[Co(en)2Cl2]+ shows optical isomerism (D and L forms) trans-[Co(en)2Cl2]+ has plane of symmetry → no optical isomerism [Co(en)3]3+ shows optical isomerism (3 bidentate ligands, no plane of symmetry)

Bonding Theories - VBT and CFT

Bonding in Coordination Compounds

Valence Bond Theory (VBT)

Metal provides empty hybrid orbitals; ligands donate electron pairs.

Common hybridizations:

Inner orbital complex (d2sp3): Uses d-orbitals of (n-1)th shell. Strong ligands cause electron pairing. Outer orbital complex (sp3d2): Uses d-orbitals of nth shell. Weak ligands, electrons not paired.

Crystal Field Theory (CFT)

Ligands treated as point charges. Degeneracy of d-orbitals is split.

Octahedral Crystal Field: Five d-orbitals split into:

• t2g (lower energy): dxy, dyz, dxz
• eg (higher energy): dx2-y2, dz2

Crystal field splitting = Delta_o (or 10Dq) Electrons fill t2g first.

Strong field ligands (large Delta_o): CN-, CO, NO2-, en, NH3 Weak field ligands (small Delta_o): I-, Br-, Cl-, F-, OH-, H2O (Spectrochemical series: I- < Br- < S2- < SCN- < Cl- < NO3- < F- < OH- < ox < H2O < NCS- < py < NH3 < en < NO2- < CN- < CO)

High Spin vs Low Spin (Octahedral, d^4 - d^7):

• Strong field ligands (large Delta_o > P, pairing energy): Low spin (more paired, fewer unpaired)
• Weak field ligands (small Delta_o < P): High spin (Hund's rule followed)

Example: [Fe(CN)6]^4- (d^6, CN- is strong): Low spin, t2g^6 eg^0, 0 unpaired → diamagnetic [Fe(H2O)6]^2+ (d^6, H2O is weak): High spin, t2g^4 eg^2, 4 unpaired → paramagnetic

Crystal Field Stabilization Energy (CFSE): CFSE = (number of electrons in t2g × (-0.4 Delta_o)) + (number of electrons in eg × (+0.6 Delta_o))

Tetrahedral Crystal Field: Splitting (Delta_t) is smaller: Delta_t = (4/9) Delta_o Ordering reversed: e (dx2-y2, dz2, lower) and t2 (higher) Most tetrahedral complexes are high-spin (Delta_t is small).

Frequently asked questions

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Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Coordination Compounds.