Catalysis by Transition Metals: Variable oxidation states are the mechanistic key. Iron (+2/+3) catalyses the Haber process for ammonia synthesis: $N_{2}$ + $3H_{2}$ → $2NH_{3}$. The iron surface adsorbs $N_{2}$ and $H_{2}$, weakening their bonds, and the metal cycles between oxidation states as intermediates form. Vanadium pentoxide ($V_{2}O_{5}$) catalyses the Contact process: $SO_{2}$ → $SO_{3}$, cycling between $V^{5+}$ and $V^{4+}$. $MnO_{2}$ catalyses decomposition of $KClO_{3}$ to release $O_{2}$. Nickel is used in the hydrogenation of vegetable oils. Platinum and palladium are used in catalytic converters (reducing CO and NOx emissions).

$KMnO_{4}$ in Analytical Chemistry: $KMnO_{4}$ is a self-indicating titrant — no external indicator needed because the endpoint is marked by the persistence of a faint pink/violet colour (excess $KMnO_{4}$). In acidic medium, it titrates: $Fe^{2+}$ to $Fe^{3+}$, oxalate ($C_{2}O_{4}^{2-}$) to C$O_{2}$, and H_{2}$$O_{2} to $O_{2}$. The stoichiometry relies on 5 moles of electrons gained per mole of $MnO_{4}^{-}$ (acidic medium), so the n-factor of $KMnO_{4}$ = 5.

Coordination Compounds in Medicine: Cisplatin (cis-[Pt($NH_{3}$)_{2}$Cl_{2}$]) — the first platinum-based anticancer drug — forms intrastrand DNA crosslinks that prevent replication in rapidly dividing cancer cells. Only the cis isomer is active; the trans isomer (transplatin) is inactive because its geometry is incompatible with intrastrand crosslink formation. EDTA (ethylenediaminetetraacetic acid, hexadentate) is used in chelation therapy to remove heavy metal ions ($Pb^{2+}$, $Hg^{2+}$) from the body by forming stable, water-soluble complexes that are excreted renally.

Coordination Compounds in Biology: Haemoglobin carries $O_{2}$ from lungs to tissues using $Fe^{2+}$ at the centre of a porphyrin ring. Carbon monoxide binds to the same $Fe^{2+}$ site with ~200× greater affinity than $O_{2}$, displacing it and causing CO poisoning. Chlorophyll absorbs red and blue light (reflecting green) through the $Mg^{2+}$ porphyrin centre, driving photosynthesis. Vitamin $B_{12}$ contains a cobalt-corrin complex; $Co^{3+}$ is essential for methyl transfer reactions in DNA synthesis. Carbonic anhydrase uses $Zn^{2+}$ to catalyse rapid interconversion of C$O_{2}$ and H_{2}$$CO_{3} in red blood cells, enabling efficient C$O_{2}$ transport.

EDTA in Industry: Food preservatives (prevents metal-catalysed oxidation of fats), complexometric titrations (hardness of water = $Ca^{2+}$ + $Mg^{2+}$ measured with EDTA), photography (fixing agent removes silver halides), and textile dyeing (prevents metal-catalysed dye degradation).

Lanthanoid Applications: Lanthanoid contraction makes 5d metals (Pt, Ir, Os) dense and hard, ideal for industrial catalysts and electronic components. Nd in $Nd_{2}Fe_{14}B$ magnets (strongest permanent magnets). La in nickel-metal hydride batteries. $Ce_{2}O_{3}$ as a catalyst in catalytic converters.