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Coordination Compounds revision notes

Every idea in the Coordination Compounds chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 29 and download the PDF.

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Werner's theory, ligands and terminology

Werner's theory: primary and secondary valence
8× asked, last 2025
primary valence=oxidation state (normally ionisable),secondary valence=coordination number\text{primary valence} = \text{oxidation state (normally ionisable)},\qquad \text{secondary valence} = \text{coordination number}
Primary: anions, ionisable unless inside [ ]. Secondary: a fixed number of ligands, non-ionisable, directional: sets the shape.
CoNH₃NH₃NH₃ClNH₃NH₃Cl⁻Cl⁻inside [ ]: 6 donor atoms = secondary valenceoutside: ionisable, Co +3 = primary
[CoCl(NHX3)X5]ClX2\ce{[CoCl(NH3)5]Cl2}: only the two outer ClX−\ce{Cl-} ionise
  • Not the complex-ion charge or the number of ligands. Primary valency is the metal's oxidation state, secondary the count of donor atoms. [CoCl(NHX3)X5]ClX2\ce{[CoCl(NH3)5]Cl2}: 3 and 6; each en adds 2, so [CoClX2(en)X2]Cl\ce{[CoCl2(en)2]Cl} is also 6.
Counting free ions: AgNO₃, conductivity, ΔTf
16× asked, last 2026
nAgCl=number of ClX− outside [ ],i=1+counter ions=ΔTfKf m[Co(NHX3)X6]ClX3 4,[CoCl(NHX3)X5]ClX2 3,[CoClX2(NHX3)X4]Cl 2 ions;[CoClX3(NHX3)X3] non-electrolyte\begin{gathered} n_{\ce{AgCl}} = \text{number of } \ce{Cl-} \text{ outside } [\ ],\qquad i = 1 + \text{counter ions} = \dfrac{\Delta T_f}{K_f\,m}\\ \ce{[Co(NH3)6]Cl3}\ 4,\quad \ce{[CoCl(NH3)5]Cl2}\ 3,\quad \ce{[CoCl2(NH3)4]Cl}\ 2\ \text{ions};\quad \ce{[CoCl3(NH3)3]}\ \text{non-electrolyte}\end{gathered}
Ligands fill CN 6 (Co³⁺, Cr³⁺) first; leftover anions go outside. More ions, more conductance. i assumes full ionisation.
ComplexCl inside [ ]Free ClX−\ce{Cl-} (mol AgCl)Ions (ii)
[Co(NHX3)X6]ClX3\ce{[Co(NH3)6]Cl3}034
[CoCl(NHX3)X5]ClX2\ce{[CoCl(NH3)5]Cl2}123
[CoClX2(NHX3)X4]Cl\ce{[CoCl2(NH3)4]Cl}212
[CoClX3(NHX3)X3]\ce{[CoCl3(NH3)3]}300 (non-electrolyte)
  • Not every Cl in the formula. Only counter ions precipitate. 20 mL of 0.01 M [CrCl(HX2O)X5]ClX2\ce{[CrCl(H2O)5]Cl2} holds 0.4 mmol free ClX−\ce{Cl-}, needing 4 mL of 0.1 M AgNOX3\ce{AgNO3}. A neutral complex gives no AgCl.
Terminology: double salt vs complex
4× asked, last 2025
Homoleptic: one kind of ligand ([Co(NHX3)X6]X3+\ce{[Co(NH3)6]^{3+}}); heteroleptic: more ([CoClX2(NHX3)X4]X+\ce{[CoCl2(NH3)4]+}). Counter ions sit outside [ ].
  • Not 'dots mean double salt'. CuSOX4 ⋅ 4 NHX3 ⋅ HX2O\ce{CuSO4.4NH3.H2O} and Fe(CN)X2 ⋅ 4 KCN\ce{Fe(CN)2.4KCN} stay as [Cu(NHX3)X4]X2+\ce{[Cu(NH3)4]^{2+}} and [Fe(CN)X6]X4−\ce{[Fe(CN)6]^{4-}}. Likewise [Fe(CN)X5NO]X2−\ce{[Fe(CN)5NO]^{2-}} is heteroleptic: count ligand kinds inside [ ] only.
Ligands and denticity
13× asked, last 2026
uni: ClX−, HX2O, NHX3, CObi: en (2 N), CX2OX4X2− (2 O), COX3X2− (2 O, or 1), gly− (N, O), dmgH− (2 N)hexa: EDTAX4− (2 N+4 O);ambidentate: NOX2X− (N or O), SCNX− (S or N), CNX− (C or N)\begin{gathered}\text{uni: } \ce{Cl-},\ \ce{H2O},\ \ce{NH3},\ \ce{CO}\\ \text{bi: en } (2\,\ce{N}),\ \ce{C2O4^{2-}}\ (2\,\ce{O}),\ \ce{CO3^{2-}}\ (2\,\ce{O},\ \text{or } 1),\ \text{gly}^-\ (\ce{N},\ \ce{O}),\ \text{dmgH}^-\ (2\,\ce{N})\\ \text{hexa: } \ce{EDTA^{4-}}\ (2\,\ce{N} + 4\,\ce{O});\qquad \text{ambidentate: } \ce{NO2-}\ (\text{N or O}),\ \ce{SCN-}\ (\text{S or N}),\ \ce{CN-}\ (\text{C or N})\end{gathered}
Ligand: Lewis base. Chelate: a polydentate ligand closing a ring. Ambidentate: two donor atoms but only one binds at a time.
N 2 H N 2 H
en: 2 N (bidentate)
O - O O - O
oxalate: 2 O
N 2 H O O -
gly⁻: N and O
O - O N O - O N O - O O - O
EDTA⁴⁻: 2 N + 4 O (hexadentate)
  • Not every lone pair. Count donor atoms that bind one metal together: EDTA uses 2 N + 4 O (one O per COO⁻). Swaps go by sites: 4 NHX3\ce{NH3} need 2 en.
  • Not every element present. A ligand binds through its donor atom: sulphate is an O-donor. In CuSO₄·5H₂O four H₂O and two sulphate O surround Cu²⁺ (CN 6); the fifth H₂O is only H-bonded.
Oxidation state and coordination number
8× asked, last 2025
x+∑qligands=charge of the entity;[Fe(HX2O)X5(NO)]X2+: NOX+, Fe(+1)x + \textstyle\sum q_{\text{ligands}} = \text{charge of the entity};\qquad \ce{[Fe(H2O)5(NO)]^{2+}}:\ \ce{NO+},\ \ce{Fe}(+1)
Neutral: H₂O, NH₃, CO, en, PPh₃. −1: halide, CN⁻, OH⁻, gly⁻. −2: C₂O₄²⁻, CO₃²⁻. EDTA −4.
SpeciesFednd^n
KX4[Fe(OH)X6]\ce{K4[Fe(OH)6]}+2d6d^{6}
KX3[Fe(OH)X6]\ce{K3[Fe(OH)6]}+3d5d^{5}
[FeOX4]X2−\ce{[FeO4]^{2-}}+6d2d^{2}
[Fe(HX2O)X5(NO)]X2+\ce{[Fe(H2O)5(NO)]^{2+}}+1d7d^{7}
Ligand charges: OHX−\ce{OH-} −1, OX2−\ce{O^{2-}} −2, HX2O\ce{H2O} 0, NO as NOX+\ce{NO+} +1.
  • Not the counter-ion count alone. Balance every charge: Fe is +2 in KX4[Fe(OH)X6]\ce{K4[Fe(OH)6]}, +3 in KX3[Fe(OH)X6]\ce{K3[Fe(OH)6]}, +6 (d2d^2) in [FeOX4]X2−\ce{[FeO4]^{2-}}. Fe(III) in [Fe(NHX3)Xx(CN)Xy]X−\ce{[Fe(NH3)_x(CN)_y]-} needs y=4y=4, so x+y=6x+y=6.

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Practise Coordination Compounds

24 more concepts in 6 topics

IUPAC nomenclature; Isomerism in coordination compounds; Valence bond theory; Crystal field theory; Magnetic moment and colour; and Carbonyls, stability and applications. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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IUPAC nomenclature

  • Naming: order and ligand names4×
  • Anionic complexes and formula writing2×

Isomerism in coordination compounds

  • Ionisation and solvate isomers6×
  • Linkage and coordination isomers5×
  • Geometrical isomers: standard counts17×
  • Optical isomers and total stereoisomers15×

Valence bond theory

  • Hybridisation: inner and outer orbital17×
  • Four-coordinate nickel: d⁸ and d¹⁰15×
  • Limitations of VBT and CFT3×

Crystal field theory

  • Splitting: octahedral and tetrahedral7×
  • High spin vs low spin (Δ vs P)17×
  • Spectrochemical series8×
  • Other factors deciding Δ11×
  • Tetrahedral complexes: always high spin11×
  • Crystal field stabilisation energy17×

Magnetic moment and colour

  • Unpaired electrons: d count first18×
  • Spin-only magnetic moment32×
  • Colour: d–d transitions and λ14×
  • Colours of common complexes7×

Carbonyls, stability and applications

  • Metal carbonyls: synergic bonding6×
  • Carbonyl structures and 18 electrons6×
  • Stability constants and the chelate effect5×
  • Uses: biology, medicine, catalysis5×
  • Uses: analysis and metallurgy6×

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