CLASS 12 CHEMISTRY · CHAPTER 5 · STUDY NOTES
Coordination Compounds
An exploration of the structure, bonding, nomenclature, and isomerism of coordination compounds, including Werner's theory, Valence Bond Theory, and Crystal Field Theory.
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Werner's Theory of Coordination Compounds
Distinguish chloride bound to cobalt from chloride outside the sphere.Bound chlorides behave differently from counterions
- Coordination sphere
- The central metal together with its directly bound ligands, enclosed in square brackets.
| Complex Formula | Primary Valence (Ionisable Cl) | Secondary Valence (Coordination No.) |
|---|---|---|
| [Co(NH3)6]Cl3 | 3 | 6 |
| [CoCl(NH3)5]Cl2 | 2 | 6 |
| [CoCl2(NH3)4]Cl | 1 | 6 |
Werner’s cold-solution silver-nitrate observations distinguished coordinated chloride from ionisable counterions. In [Co(NH₃)₆]Cl₃, all three chlorides lie outside the sphere; [Co(NH₃)₅Cl]Cl₂ has two outside; [Co(NH₃)₄Cl₂]Cl has one. Each complex has six donor neighbours and Co(III).
Werner’s primary valence corresponds to oxidation state and secondary valence to coordination number in the chapter account. Characteristic spatial arrangements explain isomers with the same empirical composition. A coordination number is not fixed universally for a metal in all compounds.
Let’s look a little closer.
Go deeper: What the evidence establishes
The original chloride test operates under stated reaction conditions. A coordinated ligand is not permanently non-ionisable in every solvent or reaction; substitution and dissociation have their own equilibria and rates.
Double salts such as carnallite dissolve into simple constituent ions. Under the comparison conditions, K₄[Fe(CN)₆] gives K⁺ and a retained complex anion rather than independently behaving free Fe²⁺ and CN⁻ ions. Werner’s structural account did not explain all colour or magnetic behaviour.
Take a moment. What do you think?
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Question
A cobalt complex has six donor neighbours including one coordinated chloride, with two chloride counterions outside. How much AgCl per mole of complex is expected in Werner’s stated test?
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Basic Definitions in Coordination Chemistry
Count donor contacts rather than separate molecules.Count donor atoms, then inspect their geometry
- Denticity
- The number of donor atoms from one ligand bound simultaneously to a specified metal centre.
Unidentate NH₃ donates through N and water through O. Ethane-1,2-diamine, en, is didentate: two N donors bind one metal and form a chelate ring. Three en ligands can therefore give coordination number six. EDTA can be hexadentate through two N and four O atoms.
An ambidentate ligand offers alternative donor atoms: SCN⁻ can bind through S or N, usually one donor at a time in the illustrated comparison. This differs from two donors of a chelating ligand binding one centre simultaneously.
Go deeper: Sphere, geometry and formal charge are different descriptions
The central metal accepts donor electron pairs in a Lewis-acid description. Coordination number counts direct metal–donor contacts; additional π bonding does not count the same donor twice. Octahedral, tetrahedral and square-planar polyhedra describe spatial arrangements.
Oxidation state follows formal ligand-charge bookkeeping. Homoleptic means one ligand kind; heteroleptic means more than one. Counterions balance the charge outside the sphere. Ligand count, coordination number, metal oxidation state and complex charge need not be the same number.
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Question
A complex contains two en ligands and two unidentate chlorido ligands. Find its ligand count and coordination number.
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Nomenclature of Coordination Compounds
Build the name from structural roles and charge accounting.A name encodes ligands, metal state and counterion
- Additive coordination name
- A systematic name specifying ligands before the central metal, with metal oxidation state and any counterion.
Name the cation before the anion for the full compound. Within a coordination entity, list ligand names alphabetically without using di-, tri- or other multiplicative prefixes to determine order. NH₃ is ammine, H₂O aqua and CO carbonyl; chlorido and cyanido are chapter ligand names.
Use di-, tri-, tetra- for simple ligand counts and bis-, tris- or related forms with suitable complex ligand names. Put the metal oxidation state in Roman numerals. An anionic entity uses an -ate metal name, including ferrate for iron; neutral and cationic entities retain the metal’s ordinary name.
Go deeper: Check a formula before trusting a name
For [Co(NH₃)₄Cl₂]Cl, x+4(0)+2(−1)=+1 gives Co(III). The name is tetraamminedichloridocobalt(III) chloride. Roman III refers to +3 at cobalt, while the sphere has +1 charge and coordination number six.
For K₃[Fe(CN)₆], the entity has −3 charge, Fe is +3 and its name is potassium hexacyanidoferrate(III). Square brackets define one entity; parentheses group polyatomic ligands or abbreviations and the superscript gives entity charge. Names encode composition but do not alone settle an isomer’s geometry.
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Question
A proposed explanation says “cobalt(III) means three ligands and +3 charge on every cobalt complex”. Correct both claims using [Co(NH₃)₄Cl₂]⁺.
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Isomerism in Coordination Compounds
Separate a changed arrangement from a changed donor connection.Same composition can mean a different arrangement
- Stereoisomerism
- Different spatial arrangements with the same composition and metal–ligand connectivity.
Square-planar MX₂L₂ permits adjacent cis and opposite trans X donors. Octahedral MX₂L₄ also permits cis/trans. Octahedral MA₃B₃ has fac when the three A donors occupy one face and mer when one A pair is opposite. Tetrahedral MX₂L₂ with unidentate ligands has no distinct cis/trans pair.
Chiral complexes have non-superimposable mirror images. Three bidentate chelates can produce opposite handedness around an octahedral centre. Rotating the viewing angle changes neither connectivity nor handedness; a schematic does not by itself determine the sign of optical rotation.
Go deeper: Four structural differences to recognise
Linkage isomerism uses a different donor atom of an ambidentate ligand. Ionisation isomerism exchanges a group inside the coordination sphere with a counterion. Solvate/hydrate isomerism changes whether solvent belongs inside the sphere or in the crystal lattice.
Coordination isomerism changes ligand ownership between complex cations and anions, such as NH₃ and CN⁻ sets associated with Co and Cr. These are structural comparisons, not proof of a particular exchange mechanism. A racemic mixture can have no net optical rotation while containing chiral entities.
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Question
An octahedral MA₃B₃ drawing places two A donors opposite each other. Could it be fac? Explain without relying on the drawing’s viewing angle.
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Valence Bond Theory (VBT)
Use the chapter’s model alongside evidence and recognise its limits.Relate the chapter’s bonding model to observed geometry
- Valence bond model
- A bonding description using combinations of metal orbitals oriented for overlap with ligand donor orbitals.
| Chapter VBT label | Donor contacts | Geometry |
|---|---|---|
| sp³ | 4 | Tetrahedral |
| dsp² | 4 | Square planar |
| d²sp³ or sp³d² | 6 | Octahedral |
In the VBT description, tetrahedral four-coordinate complexes use sp³, square-planar ones dsp², and octahedral ones d²sp³ or sp³d². “Inner” versus “outer” refers to which d set appears in the chapter’s orbital basis, not two different octahedral shapes.
Ni(II), d⁸, occurs in tetrahedral [NiCl₄]²⁻ with two unpaired electrons and square-planar [Ni(CN)₄]²⁻ with none. Co(III), d⁶, likewise illustrates high- and low-spin octahedral cases. Coordination number alone cannot select geometry or spin.
Go deeper: Hybridisation is a model, not a physical assembly
Hybrid orbitals are mathematical linear combinations used in a bonding description. Calling them non-existent physical objects should not imply that quantum-mechanical orbitals or bonding descriptions have no meaning.
VBT does not reliably calculate spectra, splitting energies, exact quantitative magnetism or thermodynamic/kinetic stability. Ligand-field and molecular-orbital treatments address covalency and bonding more fully; the chapter develops elementary CFT as another model.
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Question
Two four-coordinate Ni(II) complexes show different magnetic responses. Why is “four donors always means tetrahedral” insufficient?
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Crystal Field Theory (CFT)
Read level energies, populate orbitals, then compare splitting with pairing cost.Ligand directions split d-orbital energies
- Crystal-field splitting
- Separation of initially degenerate d-orbital energies caused by a non-spherical ligand environment in an electrostatic model.
For octahedral coordination, axis-directed d(x²−y²) and d(z²) form the higher eɡ pair; dxy, dxz and dyz form lower t₂g. Relative to the barycentre, eɡ lies +0.6Δₒ and t₂g −0.4Δₒ. Their degeneracy-weighted shifts sum to zero.
For d⁴–d⁷, competition between Δₒ and pairing energy can produce high- or low-spin populations. The orbital CFSE sum excludes pairing costs. In tetrahedral fields the e pair is lower and t₂ triplet higher; the same-metal/ligand/distance point-charge comparison gives Δt≈4Δₒ/9. Tetrahedral complexes are usually high spin.
Go deeper: An electrostatic model needs qualifications
The chapter’s spectrochemical trend includes Cl⁻ and F⁻ towards the weaker-field end, with NH₃, en, CN⁻ and CO towards stronger-field examples. It is empirical: ligand charge alone does not determine splitting. Metal identity, state and environment matter.
CFT neglects covalent metal–ligand contributions. Its success with level patterns does not prove bonds are purely ionic. The g subscript denotes inversion symmetry, absent in tetrahedral geometry; “tetrahedra lack all symmetry” would be incorrect.
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Question
For octahedral low-spin d⁶, calculate the orbital CFSE in units of Δₒ, and explain what energy contribution this number omits.
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Color and Magnetic Properties
Connect light removed from a beam with colour seen and pairing measured.Absorbed light and electron pairing give different evidence
- Absorption energy
- The photon energy hν=hc/λ that can drive an allowed electronic excitation matching an energy gap.
Octahedral Ti(III) is d¹. Absorption in a blue-green band can promote its electron from t₂g to eɡ and contribute to violet appearance. Near 498 nm, hc/λ≈2.490 eV. The colour transmitted is not simply the colour absorbed; real spectra involve finite bands and transition rules.
Replacing waters with didentate en around Ni²⁺ changes its ligand environment and qualitative colour while retaining six donor contacts. Ruby and emerald can both contain Cr³⁺ yet have different host-dependent absorption. Colour alone does not prove a unique oxidation state or spin count.
Go deeper: Magnetism is independent evidence
The spin-only estimate μ=√[n(n+2)] BM uses the number of unpaired electrons, not the total d count. Octahedral d⁶ can have four unpaired in high spin or none in low spin; orbital contributions and other effects can change observed moments.
Neither “no ligands means every substance is colourless” nor “all complex colour is d–d” is general. Solid environments, charge transfer and other excitations matter. The simplified photon control illustrates one chosen gap rather than a full measured absorption spectrum.
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Question
Compare photon energies at 400 nm and 600 nm using hc≈1240 eV·nm. Which is larger, and does that alone determine the observed solution colour?
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Metal Carbonyls and Applications
Follow two electronic contributions, then connect coordination with applications.Two-way electron donation strengthens metal–carbon bonding
- Synergic carbonyl bonding
- Coupled CO-to-metal σ donation and metal-to-CO π backbonding that reinforce the metal–carbon bond.
CO binds through carbon. Its carbon lone pair can donate into an available metal orbital; filled metal orbitals can back-donate into CO π*. These contributions strengthen metal–carbon bonding and backbonding tends to weaken C–O relative to free CO. The arrows represent contributions to bonding, not a circulating ionic current.
Ni(CO)₄ is tetrahedral, Fe(CO)₅ trigonal bipyramidal and Cr(CO)₆ octahedral. Neutral CO gives formal metal state zero in these neutral molecules. Mn₂(CO)₁₀ has a Mn–Mn bond and ten terminal CO groups; the chapter’s bridged Co₂(CO)₈ structure has six terminal and two bridging CO groups.
Go deeper: Applications depend on a complete chemical environment
Coordination contributes to chlorophyll’s light capture with Mg, haem oxygen transport with Fe and vitamin B₁₂ chemistry with Co. EDTA metal binding supports water-hardness estimation. Complex formation also assists metal extraction, purification, electroplating, photographic fixing and catalytic processes under controlled conditions.
The chapter’s nickel-carbonyl purification route and cyanide extraction chemistry are industrial examples, not student preparation instructions. Carbonyls and metal complexes may be hazardous. Medical chelation and platinum-compound applications are contextual chemistry; do not infer a self-treatment or that all platinum compounds have the same biological action.
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Question
A Co₂(CO)₈ structure has two bridging carbonyl groups. How many terminal carbonyl groups must it have, and do the bridging groups count twice towards its molecular formula?
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