Coordination Compounds

Unit 5: Coordination Compounds

Introduction

Coordination compounds are compounds in which a central metal atom or ion is bonded to a number of ions or molecules called ligands through coordinate (dative) bonds. Examples include [Fe(CN)6]³-, haemoglobin (Fe complex), chlorophyll (Mg complex), and Cisplatin (anticancer drug). Werner's coordination theory laid the foundation for this branch.

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Werner's Theory

1. 1.Alfred Werner (1893) proposed that metal ions in coordination compounds have:
2. 2.Primary valence (ionisable/oxidation state): Satisfied by anions; gives the overall charge.
3. 3.Secondary valence (non-ionisable/coordination number): Satisfied by neutral molecules or anions directly bonded to the metal; defines the coordination sphere.

Example: CoCl3.6NH3 (now [Co(NH3)6]Cl3): Co has primary valence 3 (from 3 Cl-) and secondary valence 6 (from 6 NH3).

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Important Definitions

• Central atom/ion: The metal atom/ion at the centre of the complex.
• Ligands: Atoms, ions, or molecules that donate electron pairs to the central metal (Lewis bases). Examples: Cl-, NH3, H2O, CN-, en.
• Coordination number (CN): Total number of coordinate bonds formed by the central atom with ligands.
• Coordination sphere: Central metal + ligands, written in square brackets.
• Counter ions: Ions outside the square bracket that balance charge.

• Types of Ligands:
• Monodentate: One donor atom (H2O, NH3, Cl-, CN-)
• Bidentate: Two donor atoms (en = ethylenediamine, ox²- = oxalate)
• Polydentate: More than two donor atoms (EDTA⁴- is hexadentate)
• Ambidentate: Can donate through two different atoms (NO2- via N or O; SCN- via S or N)
• Chelate: A complex where polydentate ligands form rings with the central metal. More stable (chelate effect).

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IUPAC Nomenclature

1. 1.Rules:
2. 2.Name cation before anion (as in salts).
3. 3.In the complex ion: ligands named before the metal.
4. 4.Anionic ligands end in -o (Cl- = chlorido, CN- = cyanido, NO2- = nitrito).
5. 5.Neutral ligands use their own names except: H2O = aqua, NH3 = ammine, CO = carbonyl, NO = nitrosyl.
6. 6.Greek prefixes (di, tri, tetra...) for simple ligands; bis, tris, tetrakis for complex names.
7. 7.Metal: in anionic complex, Latin name + ate suffix (Fe → ferrate, Cu → cuprate, Cr → chromate); in cation/neutral, IUPAC name.
8. 8.Oxidation state in Roman numerals in parentheses.

Example: [Co(NH3)4Cl2]Cl
Name: Tetraamminedichloridocobalt(III) chloride

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Isomerism in Coordination Compounds

• 1. Structural Isomerism:
• Ionisation isomerism: [Co(NH3)5Br]SO4 and [Co(NH3)5(SO4)]Br
• Linkage isomerism: [Co(NH3)5(NO2)]2+ (N-bonded) vs [Co(NH3)5(ONO)]2+ (O-bonded)
• Coordination isomerism: Ligands exchanged between cation and anion complex
• Solvate isomerism: [Cr(H2O)6]Cl3 vs [Cr(H2O)5Cl]Cl2.H2O

• 2. Stereoisomerism:
• Geometric (cis-trans) isomerism: In square planar MA2B2 and octahedral MA4B2 or MA3B3 complexes
• Optical isomerism: Non-superimposable mirror images (chiral); common in octahedral complexes with bidentate ligands

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Bonding Theories

• Valence Bond Theory (VBT):
• Metal ion hybridises its orbitals and accepts electron pairs from ligands.
• For CN-6: d^2sp3 (inner sphere — uses (n-1)d orbitals) or sp^3d2 (outer sphere — uses nd orbitals)
• For CN=4: sp³ (tetrahedral) or dsp² (square planar)
• Inner orbital complexes: low spin, more covalent character; Outer orbital: high spin.

• Crystal Field Theory (CFT):
• Ligands treated as point charges that create an electrostatic field.
• In an octahedral field, the 5 d-orbitals split into:
• t2g (dxy, dyz, dxz) — lower energy (3 orbitals)
• eg (dx2-y2, dz2) — higher energy (2 orbitals)
• Crystal field splitting energy (delta_o or 10Dq): Energy difference between t2g and eg.
• Strong field ligands (large delta_o): CN-, CO, NO2-, en → force electron pairing → low spin.
• Weak field ligands (small delta_o): Cl-, Br-, I-, F-, H2O → electrons occupy all orbitals before pairing → high spin.

Spectrochemical series (increasing field strength):
I- < Br- < S²- < SCN- < Cl- < NO3- < F- < OH- < H2O < NCS- < py < NH3 < en < NO2- < CN- < CO

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Stability of Complexes

• Depends on nature of metal and ligand, charge on metal ion, and chelate effect.
• Chelate effect: Chelated complexes (with polydentate ligands) are more stable than analogous monodentate complexes due to increased entropy (more particles released when polydentate ligands replace monodentate).

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Applications of Coordination Compounds

• Medical: Cisplatin [PtCl2(NH3)2] — anticancer drug; EDTA — treatment of lead poisoning; Vitamin B12 (Co complex)
• Metallurgy: Extraction of gold (NaCN forms [Au(CN)2]- complex); Purification of nickel by Mond process ([Ni(CO)4])
• Agriculture: Micronutrient transport in plants
• Photography: Na2S2O3 (hypo) dissolves unexposed AgBr as [Ag(S2O3)2]³-

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Worked Examples

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