Coordination Compounds: The JEE Master List
A curated list of the highest-yield coordination chemistry reactions, covering colored complexes, qualitative analysis, metallurgy, and photography applications.
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1. Colored Complexes & Precipitates
Prussian Blue
$$\ce{4Fe^3+ + 3[Fe(CN)6]^4- -> Fe4[Fe(CN)6]3 v}$$
Reaction of Iron(III) (Ferric) with Potassium ferrocyanide yields an intense Prussian Blue precipitate.
Turnbull's Blue
$$\ce{3Fe^2+ + 2[Fe(CN)6]^3- -> Fe3[Fe(CN)6]2 v}$$
Reaction of Iron(II) (Ferrous) with Potassium ferricyanide yields a Turnbull's Blue precipitate.
Blood Red Complex
$$\ce{Fe^3+ + SCN- -> [Fe(SCN)]^2+}$$
Ferric ions react with thiocyanate ions to form a characteristic Blood Red soluble complex.
Brown Ring Complex
$$\ce{[Fe(H2O)6]^2+ + NO -> [Fe(H2O)5(NO)]^2+ + H2O}$$
Nitrate confirmatory test. Forms a Brown complex. Note: The oxidation state of Fe here is +1.
Nickel DMG Complex
$$\ce{Ni^2+ + 2DMG -> [Ni(DMG)2] v + 2H+}$$
Nickel(II) reacts with dimethylglyoxime (DMG) in slightly basic medium to give a Rosy Red precipitate. Forms square planar complex with H-bonding.
Nitroprusside Test
$$\ce{S^2- + [Fe(CN)5(NO)]^2- -> [Fe(CN)5(NOS)]^4-}$$
Sodium nitroprusside reacts with sulfide ions to form a brilliant Purple/Violet complex.
Chocolate Brown Ppt
$$\ce{2Cu^2+ + [Fe(CN)6]^4- -> Cu2[Fe(CN)6] v}$$
Cupric ions react with potassium ferrocyanide to form a Chocolate Brown precipitate of Cupric ferrocyanide.
Zinc Ferrocyanide
$$\ce{2Zn^2+ + K4[Fe(CN)6] -> K2Zn3[Fe(CN)6]2 v + 4K+}$$
Zinc ions react with ferrocyanide to form a White precipitate.
Cobalt Thiocyanate
$$\ce{Co^2+ + 4SCN- -> [Co(SCN)4]^2-}$$
Cobalt(II) forms a beautiful Deep Blue colored tetrahedral complex with excess thiocyanate.
2. Ligand Substitution & Solubilization
Deep Blue Cu Ammine
$$\ce{[Cu(H2O)4]^2+ + 4NH3 -> [Cu(NH3)4]^2+ + 4H2O}$$
Addition of excess aqueous ammonia to Cu(II) solutions replaces water ligands, forming a Deep Blue square planar complex.
Silver Chloride Dissolution
$$\ce{AgCl(s) + 2NH3(aq) -> [Ag(NH3)2]+ + Cl-}$$
Insoluble white precipitate of AgCl dissolves in aqueous ammonia forming the soluble Diamminesilver(I) complex.
Silver Bromide Dissolution
$$\ce{AgBr(s) + 2S2O3^2- -> [Ag(S2O3)2]^3- + Br-}$$
Insoluble AgBr dissolves in sodium thiosulfate (hypo) to form a soluble complex. Used in photography.
Aluminium Amphoterism
$$\ce{Al(OH)3(s) + OH- -> [Al(OH)4]-}$$
White gelatinous precipitate of Aluminium hydroxide dissolves in excess NaOH forming a soluble complex.
Zinc Amphoterism
$$\ce{Zn(OH)2(s) + 2OH- -> [Zn(OH)4]^2-}$$
White precipitate of Zinc hydroxide dissolves in excess strong base to form soluble tetrahydroxozincate(II).
EDTA Hardness Complex
$$\ce{Ca^2+ + EDTA^4- -> [Ca(EDTA)]^2-}$$
Hexadentate EDTA ligand wraps around Calcium/Magnesium ions to form highly stable octahedral complexes. Used to estimate water hardness.
3. Metallurgy Applications
MacArthur-Forrest Cyanide Process
$$\ce{4Au + 8CN- + 2H2O + O2 -> 4[Au(CN)2]^- + 4OH-}$$
Leaching of gold ore. The oxidation of gold by air is coupled with complexation by cyanide to form a soluble dicyanoaurate(I) complex.
Zinc Cementation (Recovery)
$$\ce{2[Au(CN)2]^- + Zn -> [Zn(CN)4]^2- + 2Au v}$$
More electropositive Zinc displaces gold from its cyanide complex, precipitating pure gold.
Mond Process for Nickel
$$\ce{Ni + 4CO ->[330 K] Ni(CO)4 ->[450 K] Ni + 4CO ^}$$
Impure Nickel reacts with carbon monoxide to form volatile tetracarbonylnickel(0), leaving impurities behind. It is then thermally decomposed to yield pure Ni.
4. Syntheses, Isomers & Catalysts
Synthesis of Cis-platin
$$\ce{[PtCl4]^2- + 2NH3 -> cis-[PtCl2(NH3)2] + 2Cl-}$$
Relies on the Trans Effect. $Cl^-$ has a higher trans-directing effect than $NH_3$, forcing the second $NH_3$ to substitute cis to the first one. Used as an anti-cancer drug.
Synthesis of Trans-platin
$$\ce{[Pt(NH3)4]^2+ + 2Cl- -> trans-[PtCl2(NH3)2] + 2NH3}$$
Opposite starting material. The trans effect of $Cl^-$ forces the second $Cl^-$ to substitute trans to the first one.
Wilkinson's Catalyst
$$\ce{[RhCl(PPh3)3]}$$
Chloridotris(triphenylphosphine)rhodium(I). A crucial homogeneous coordination catalyst used for the selective hydrogenation of alkenes.
Ziegler-Natta Catalyst
$$\ce{TiCl4 + Al(C2H5)3}$$
A heterogeneous coordination catalyst system used globally for the synthesis of high-density polyethylene (HDPE).
Synergic Bonding in Carbonyls
$$\ce{M(d\pi) -> CO(\pi^*)}$$
Not a full reaction, but a vital concept: Metal donates electron density back into the empty anti-bonding $\pi^*$ orbital of CO, strengthening the M-C bond and weakening the C-O bond.
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