When a coordination compound dissolves in water, only the ions outside the coordination sphere contribute to electrical conductance. The complex ion itself (the coordination sphere) acts as a single ion. Molar conductance values distinguish types: 0 ions (non-electrolyte): conductance ≈ 0. [Co(NH3)3Cl3]. 2 ions (1:1 electrolyte): ~100-130 S cm²/mol. [Co(NH3)4Cl2]Cl. 3 ions (1:2 or 2:1): ~230-270 S cm²/mol. [Co(NH3)5Cl]Cl2. 4 ions (1:3): ~350-390 S cm²/mol. [Co(NH3)6]Cl3. This is exactly how Werner determined the coordination sphere: by measuring conductance and comparing with expected ion counts. Werner (1893) revolutionised inorganic chemistry with his coordination theory.

Werner (1893) proposed: (1) Metal has primary valence (oxidation state) and secondary valence (coordination number). (2) Secondary valence is always satisfied by ligands directly bonded to metal (coordination sphere). (3) Primary valence may be satisfied by counter-ions outside coordination sphere or by ligands inside. (4) Coordination number is fixed for a given metal ion. Evidence for Werners theory: conductance measurements (different number of ions), precipitation with AgNO3 (only Cl⁻ outside sphere precipitates immediately), isomerism (cis-trans, optical). Werner experimentally verified his theory using Co(III) complexes with NH3 and Cl⁻ ligands — the exact complexes in this question. He won Nobel Prize in Chemistry in 1913 — the first inorganic chemist to receive it.

Ligands: ions or molecules that donate electron pairs to central metal. Monodentate (one donor atom): Cl⁻, Br⁻, F⁻, CN⁻, NCS⁻, OH⁻, H2O, NH3, CO, NO. Bidentate (two donor atoms): en (ethylenediamine, N-N), ox²⁻ (oxalate, O-O), bipy (bipyridyl, N-N), phen (phenanthroline, N-N), acac⁻ (acetylacetonate, O-O). Tridentate: dien (diethylenetriamine). Tetradentate: trien. Hexadentate: EDTA (four O, two N donor atoms — forms 5 five-membered rings with metal → exceptionally stable). Ambidentate: can coordinate through either of two atoms. SCN⁻ (S or N), NO2⁻ (N or O). Give rise to linkage isomerism. Bridging ligands: bond to two metal centres simultaneously. μ-OH, μ-Cl, μ-CO.

Structural isomers: (1) Ionisation isomers: [Co(NH3)5Br]SO4 vs [Co(NH3)5SO4]Br. Different ions in vs outside sphere. (2) Hydrate isomers: [Cr(H2O)6]Cl3, [CrCl(H2O)5]Cl2·H2O, [CrCl2(H2O)4]Cl·2H2O. (3) Linkage isomers: [Co(NH3)5NO2]²⁺ (nitro, N-bonded) vs [Co(NH3)5ONO]²⁺ (nitrito, O-bonded). (4) Coordination position isomers. Stereoisomers: (5) Geometric (cis-trans): [PtCl2(NH3)2] — cisplatin (cis, anticancer drug) vs transplatin (inactive). For octahedral [MA3B3]: facial (fac, all A mutually cis) and meridional (mer, A in a plane). (6) Optical isomers: [Co(en)3]³⁺, [CoCl2(en)2]⁺ (cis-form only). Non-superimposable mirror images (enantiomers), rotate polarised light.

Rules: name cation before anion. Within complex: ligands alphabetically, then metal with OS in parentheses. Anionic ligands: -o (Cl⁻=chlorido, CN⁻=cyanido, OH⁻=hydroxido, O²⁻=oxido, NO2⁻=nitrito, SCN⁻=thiocyanato). Neutral: H2O=aqua, NH3=ammine, CO=carbonyl, NO=nitrosyl. Prefixes: di, tri, tetra, penta, hexa for simple ligands; bis, tris, tetrakis for complex ligands. Anionic complexes: metal gets -ate suffix. Fe→ferrate, Cr→chromate, Co→cobaltate, Pt→platinate, Ni→nickelate, Cu→cuprate, Ag→argentate, Au→aurate. Examples: [Co(NH3)5Cl]Cl2 = pentaamminechloridocobalt(III) chloride. K4[Fe(CN)6] = potassium hexacyanidoferrate(II). [Cr(en)3]³⁺ = tris(ethane-1,2-diamine)chromium(III) cation.

Stability constant Kf = [MLn] / ([M][L]^n). Larger Kf = more stable complex. Factors: (1) Nature of metal: high charge, small size, d-block metals form more stable complexes (higher charge density). (2) Nature of ligand: strong field ligands (CN⁻, en) > weak field (Cl⁻, H2O). Chelate effect: polydentate > monodentate (entropy advantage). (3) CFSE: d³, d⁶(low spin), d⁸ configurations have maximum CFSE → maximum stability. (4) Hard-Soft Acid-Base (HSAB): hard metal + hard ligand OR soft + soft = stable. Hard: Fe³⁺, Co³⁺, Cr³⁺, Al³⁺ + hard ligands (F⁻, OH⁻, NH3, H2O). Soft: Hg²⁺, Pt²⁺, Pd²⁺, Au⁺ + soft ligands (CN⁻, CO, I⁻, PR3). Irving-Williams series: Mn²⁺ < Fe²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺ > Zn²⁺.

Haemoglobin: Fe²⁺ in porphyrin ring (tetradentate) with globin protein (N donor). Sixth coordination site binds O2 reversibly. Myoglobin: similar, stores O2 in muscles. CO poisoning: CO binds Fe²⁺ ~200× stronger than O2 → carboxyhemoglobin → no O2 transport. Chlorophyll: Mg²⁺ in porphyrin. Light absorption for photosynthesis. Vitamin B12 (cobalamin): Co³⁺ in corrin ring. Essential for DNA synthesis, nerve function. Cisplatin [cis-Pt(NH3)2Cl2]: anticancer drug. Cross-links DNA strands in rapidly dividing cancer cells → prevents replication. EDTA: chelates heavy metals (Pb²⁺, Hg²⁺ poisoning treatment) by forming stable complexes for urinary excretion. Carbonic anhydrase: Zn²⁺ enzyme, converts CO2 + H2O → H2CO3 in blood at 10⁶ reactions/s.

Electroplating: metal complexes (e.g., [Au(CN)2]⁻ for gold plating) give smoother, more uniform deposits than simple salts. Photography: [Ag(S2O3)2]³⁻ (fixer) dissolves unexposed AgBr. Analytical chemistry: EDTA titrations (determine hardness of water — Ca²⁺ and Mg²⁺ concentrations). Colorimetry: Fe³⁺ + SCN⁻ → blood-red [Fe(SCN)]²⁺ (qualitative test for Fe³⁺). Extraction metallurgy: Au extracted with NaCN forming [Au(CN)2]⁻; Ag similarly. Ni extracted as [Ni(CO)4] (volatile, Mond process) → decomposed at higher T to give pure Ni. Catalysis: Wilkinson catalyst [RhCl(PPh3)3] for hydrogenation. Wacker process [PdCl2]: CH2=CH2 + O2 → CH3CHO. Ziegler-Natta [TiCl4/Al(C2H5)3]: stereospecific polymerisation of alkenes.