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Unit 5: Coordination Compounds

5.1 Introduction and Basic Terms

Coordination compounds consist of a central metal atom or ion connected to a surrounding array of molecules or anions. These surrounding species are called ligands. Unlike normal salts, coordination compounds retain their identity even in solution.

  • Central Atom/Ion: The cation or neutral atom to which ligands are attached. Acts as a Lewis acid (electron pair acceptor).
  • Ligand: Ions or molecules bound to the central atom/ion. They act as Lewis bases (electron pair donors).
    • Unidentate: \(Cl^-, H_2O, NH_3\)
    • Didentate: ethylenediamine (en), oxalate (\(C_2O_4^{2-}\))
    • Polydentate: EDTA (hexadentate)
  • Coordination Number (CN): The total number of ligand donor atoms to which the metal is directly bonded.

5.2 IUPAC Nomenclature of Coordination Compounds

Rules for naming coordination compounds:

  1. The cation is named first in both positively and negatively charged coordination entities.
  2. The ligands are named in alphabetical order before the name of the central atom/ion.
  3. Names of anionic ligands end in -o (e.g., chlorido, cyanido), neutral ligands have their normal names (exceptions: \(H_2O\) is aqua, \(NH_3\) is ammine, \(CO\) is carbonyl).
  4. Prefixes mono, di, tri, etc., indicate the number of individual ligands. When the names of ligands include a numerical prefix, terms like bis, tris, tetrakis are used.
  5. Oxidation state of the metal is indicated by a Roman numeral in parentheses.
  6. If the complex ion is an anion, the name of the metal ends with the suffix -ate (e.g., ferrate, cobaltate).

5.3 Isomerism in Coordination Compounds

Types of stereoisomerism:

  • Geometrical Isomerism: Found mainly in square planar (CN=4) and octahedral (CN=6) complexes. Leads to cis (similar groups adjacent) and trans (similar groups opposite) isomers.
  • Optical Isomerism: Observed in octahedral complexes involving didentate ligands (e.g., \([Co(en)_3]^{3+}\)). Isomers are non-superimposable mirror images (d and l forms).

Types of structural isomerism:

  • Linkage Isomerism: Arises in complexes containing ambidentate ligands (e.g., \(NO_2^-\), \(SCN^-\)).
  • Coordination Isomerism: Interchange of ligands between cationic and anionic entities of different metals.
  • Ionisation Isomerism: The counter ion in a complex salt acts as a ligand and the ligand acts as a counter ion.
  • Solvate/Hydrate Isomerism: Involves exchange of solvent molecules (e.g., water) between the coordination sphere and the crystal lattice.

5.4 Bonding in Coordination Compounds

Werner’s Theory

Metals possess two types of valencies: primary valency (ionisable, satisfies oxidation state) and secondary valency (non-ionisable, satisfies coordination number and dictates the spatial arrangement of ligands).

Valence Bond Theory (VBT)

The central metal ion uses its (n-1)d, ns, np or ns, np, nd orbitals for hybridization to yield a set of equivalent orbitals of definite geometry (e.g., octahedral, square planar).

  • Inner orbital complex: Uses (n-1)d orbitals (e.g., \(d^2sp^3\)). Usually form with strong field ligands; low spin.
  • Outer orbital complex: Uses nd orbitals (e.g., \(sp^3d^2\)). Usually form with weak field ligands; high spin.
3d ⥮ ⥮ ⥮ 4s 4p d²sp³ hybridization (Inner orbital, Diamagnetic) xx xx xx xx xx xx VBT representation of [Co(NH₃)₆]³⁺

Crystal Field Theory (CFT)

Consider the ligands as point charges. The five degenerate d-orbitals split into two sets of orbitals of different energies in the presence of the ligand field.

  • In an octahedral field: Energy of the \( e_g \) set (\( d_{x^2-y^2} \), \( d_{z^2} \)) is raised while the energy of the \( t_{2g} \) set (\( d_{xy}, d_{yz}, d_{zx} \)) is lowered.
    • Difference in energy is \( \Delta_o \) (Crystal Field Splitting Energy).
Energy Free Metal Ion Average Energy in Spherical Field e_g t2g Δo +0.6Δo -0.4Δo Figure 5.1: d-orbital splitting in an octahedral crystal field
  • Spectrochemical Series: Arrangement of ligands in increasing order of crystal field splitting strength. \[ I^- < Br^- < SCN^- < Cl^- < F^- < OH^- < C_2O_4^{2-} < H_2O < NCS^- < EDTA^{4-} < NH_3 < en < CN^- < CO \]

Colour: Due to d-d transitions of unpaired electrons in the visible region when light falls on the complex.

5.5 Importance of Coordination Compounds

  • Extraction of metals: Ag and Au are extracted using cyanide complexes \([Ag(CN)_2]^-\).
  • Estimation of hardness: Ca\(^{2+}\) and Mg\(^{2+}\) can be estimated by complexometric titration with EDTA.
  • Biological systems: Chlorophyll (Mg complex), Haemoglobin (Fe complex), Vitamin B\(_{12}\) (Co complex).
  • Catalysts: Wilkinson’s catalyst \([RhCl(PPh_3)_3]\) used for hydrogenation of alkenes.
  • Medicine: Cisplatin \([Pt(NH_3)_2Cl_2]\) is used in cancer therapy.

Competency-Based Questions (CBQs)

Q1. (CBSE 2024 Pattern) The magnetic moment of \([MnCl_4]^{2-}\) is 5.9 BM whereas for \([Mn(CN)_6]^{3-}\) it is 2.8 BM. Using Valence Bond Theory, explain this difference in magnetic behavior and predict the hybridization in both the complexes.


Answer: For \([MnCl_4]^{2-}\): Oxidation state of Mn is +2. Electronic configuration of Mn\(^{2+}\) is \(3d^5\). Chloride (\(Cl^-\)) is a weak field ligand. It does not force the pairing of electrons against Hund’s rule. So, there are 5 unpaired electrons in the 3d orbitals. Using the formula \(\mu = \sqrt{n(n+2)}\): \[ \mu = \sqrt{5(5+2)} = \sqrt{35} \approx 5.92 , \text{BM} \] The hybridization involves one 4s and three 4p orbitals, forming \(sp^3\) hybridization to accommodate four \(Cl^-\). The complex is tetrahedral and highly paramagnetic.

For \([Mn(CN)_6]^{3-}\): Oxidation state of Mn is +3 (since \(x - 6 = -3 \rightarrow x = +3\)). Electronic configuration of Mn\(^{3+}\) is \(3d^4\). Cyanide (\(CN^-\)) is a strong field ligand. It forces the pairing of electrons in the 3d orbitals to make room for hybridization. Two electrons pair up while the remaining two remain unpaired (so n = 2). \[ \mu = \sqrt{2(2+2)} = \sqrt{8} \approx 2.8 , \text{BM} \] Two 3d orbitals are vacant. The hybridization is \(d^2sp^3\) (inner orbital complex). The shape is octahedral and it is paramagnetic but less than \([MnCl_4]^{2-}\).

Q2. (CBSE 2023) Draw the structures of optical isomers of the coordination complex: \( [Co(en)_3]^{3+} \).


Answer: The complex \( [Co(en)_3]^{3+} \) (where ‘en’ stands for ethylenediamine, a didentate ligand) forms an octahedral geometry. As it lacks a plane of symmetry, it exists in two non-superimposable mirror image forms, known as the d- (dextro) and l- (laevo) enantiomers.

3+ Co N N N N N N en en en d-isomer 3+ Co N N N N N N en en en l-isomer

Q3. (Sample Paper 2023) Explain on the basis of crystal field theory why \([Co(NH_3)_6]^{3+}\) is an inner orbital and diamagnetic complex, while \([CoF_6]^{3-}\) is an outer orbital and paramagnetic complex.


Answer: Both complexes contain Cobalt in +3 oxidation state, meaning the electronic configuration of \(Co^{3+}\) is \(3d^6\). According to CFT, the 3d orbitals split into lower energy \(t_{2g}\) and higher energy \(e_g\) levels.

  • In the presence of \(NH_3\) (a strong field ligand), the crystal field splitting energy (\(\Delta_o\)) is greater than the pairing energy ((P)), i.e. \(\Delta_o > P\). Therefore, electrons prefer to pair up in the lower energy \(t_{2g}\) orbitals rather than jumping to \(e_g\). The configuration becomes \(t_{2g}^6 e_g^0\). Because all electrons are paired, it is a diamagnetic, low-spin inner orbital complex.
  • In the presence of \(F^-\) (a weak field ligand), the crystal field splitting energy is less than the pairing energy, \(\Delta_o < P\). Electrons distribute to both \(t_{2g}\) and \(e_g\) orbitals before pairing. The configuration is \(t_{2g}^4 e_g^2\). Because it has four unpaired electrons, it is an outer orbital, highly paramagnetic high-spin complex.