13.3 Werner Theory, Isomerism, IUPAC Names and VBT of Complexes

13.3.1 Werner's coordination theory ★★

• A metal has a primary valence (ionisable; today the oxidation state, satisfied by anions) and a secondary valence (non-ionisable; today the coordination number, satisfied by ligands with a fixed geometry).SN
• Proof from the cobalt(III) ammines: AgNO₃ precipitates only the chloride outside the bracket, and molar conductivity counts the ions.

• Geometry from isomer counts: MA₄B₂ gives only two isomers. A hexagon or a trigonal prism would give three; only the octahedron gives two. The resolution of the all-inorganic “hexol” [Co{(OH)₂Co(NH₃)₄}₃]⁶⁺ (1914) showed that chirality needs no carbon.SN
• Two isomers of CoCl₃·4NH₃: praseo (green) = trans, violeo (violet) = cis; both are achiral (MA₄B₂) IFoS 2021 Q6(c) 10m.SN

13.3.2 IUPAC names ★★★

1. Cation before anion; the number of counter-ions is not stated.
2. Inside the bracket, ligands first in alphabetical order (ignore multiplying prefixes), then the metal with its oxidation number in Roman numerals.SN
3. Anionic ligands end in -ido, -ato, -ito (chlorido, cyanido, hydroxido, oxido, oxalato, nitrito); neutral ones keep their names except aqua, ammine, carbonyl, nitrosyl.SN
4. di-, tri-, tetra- for simple ligands; bis-, tris-, tetrakis- for names that already contain a number or are complex: bis(ethane-1,2-diamine), tetrakis(pyridine).
5. An anionic complex ends in -ate: ferrate, cuprate, argentate, aurate, plumbate, stannate; cobaltate, platinate, zincate.SN
6. Bridging ligands carry μ and are cited first; κ shows the donor atom (nitrito-κN = nitro, nitrito-κO); η shows hapticity.

• Check the oxidation state from charge balance before writing the Roman numeral: in [(NH₃)₄Co(μ-NH₂)(μ-O₂)Co(NH₃)₄]⁴⁺, 2x − 1 + q(O₂) = +4. With superoxide (q = −1) both Co are +3: μ-amido-μ-superoxido-bis[tetraamminecobalt(III)] IFoS 2025 Q7(b) 10m CSE 2024 Q5(c) 10m CSE 2023 Q6(c) 10m.SN
• Drawing from a name: potassium trans-dichloridodinitrito-κN-platinate(II) is square planar with Cl trans to Cl; Mohr's salt (NH₄)₂Fe(SO₄)₂·6H₂O is a double salt containing [Fe(H₂O)₆]²⁺, not a single complex ion IFoS 2021 Q8(a) 15m.

13.3.3 Structural isomerism ★★

• Nitro–nitrito: [Co(NH₃)₅Cl]Cl₂ + NaNO₂ in dilute NH₃ gives first the red nitrito (O-bonded, kinetic) isomer, which on standing or warming changes to the yellow nitro (N-bonded, stable) isomer; UV light turns it back. IR: nitro 1430, 1315 cm⁻¹; nitrito 1460, 1065 cm⁻¹ Geo 2025 Q1(b) 5m CSE 2024 Q7(b)(ii) 5m.SN
• Which linkage isomer is stable? The soft C end of CN⁻ goes to the soft metal centre (the one carrying five CN⁻, “symbiosis”), the hard N end to the hard Cr(NH₃)₅ centre: [(H₃N)₅Cr–NC–Cr(CN)₅] is the stable isomer CSE 2022 Q8(a) 10m.SN

13.3.4 Four-coordinate complexes ★★

• Tetrahedral MA₂B₂: no geometrical isomers (all positions adjacent). Mabcd is chiral only if all four are different.
• Square planar: Ma₂b₂ and Ma₂bc give cis and trans (2 each); Mabcd gives 3 (fix a; its trans partner is b, c or d). Planar point ligands give no optical isomers Geo 2022 Q1(c) 5m.SN
• Chiral square-planar complex: [Pt(meso-stilbenediamine)(isobutylenediamine)]²⁺ has no mirror plane because the two chelates are unsymmetrical; Mills and Quibell (1935) resolved it, which proved that Pt(II) is planar (a tetrahedral ion of this kind would have a mirror plane) CSE 2022 Q7(a) 10m.SN

Kurnakov test: thiourea has a strong trans effect. In the cis isomer each tu that enters sits trans to an amine and labilises it, so all four ligands are replaced. In the trans isomer each tu sits trans to the other tu or Cl, the amines (trans to each other) survive, and the product is the sparingly soluble trans-[PtA₂(tu)₂]Cl₂. A second chemical test: only the cis isomer can be converted into a chelate (with oxalate) Geo 2023 Q2(b) 10m.SN