platinum-group metals
/ PGMs: pee-jee-emz /
Cut the bottom-right corner out of the transition block and you find six rare, precious, beautiful metals huddled together: ruthenium, rhodium, palladium (the 4d trio) and osmium, iridium, platinum (the 5d trio). These are the platinum-group metals, or PGMs. They are scarce, expensive, dense, and — most famously — chemically aloof, resisting corrosion and tarnish even at high temperatures. They are the metals in your car's catalytic converter, in fine jewellery, and in many of the catalysts that make modern chemicals and drugs.
What unites them is a kind of chemical reserve combined with extraordinary catalytic power. Because they are late, heavy transition metals, their d electrons are held tightly and they sit near the bottom of the reactivity scale (platinum and gold are 'noble' for that reason). Yet that same restraint makes them superb catalysts: a platinum or palladium surface can grab a molecule like hydrogen or carbon monoxide just firmly enough to weaken its bonds and let it react, then release the product — they bind reactants reversibly, the hallmark of a good catalyst. The 4d and 5d congeners within the group are near-twins in size (thanks to the lanthanide contraction), and as heavy metals they readily reach higher oxidation states, form low-spin complexes, and favor square-planar geometry in their d8 ions (platinum(II) and palladium(II)).
The PGMs are technological treasures. Platinum, palladium and rhodium in catalytic converters scrub the exhaust of nearly every petrol car, turning poisonous CO and nitrogen oxides into harmless gases — which is why these metals are mined, recycled and traded at high prices. Palladium and platinum catalyze the cross-coupling reactions that build pharmaceuticals (Nobel-winning chemistry), and Wilkinson's rhodium catalyst hydrogenates alkenes in solution. Platinum even fights cancer: the drug cisplatin, a simple square-planar platinum(II) complex, binds DNA and stops tumour cells dividing. The PGMs show the upper reaches of the d-block at their most useful — unreactive as metals, indispensable as catalysts.
A three-way catalytic converter uses platinum, palladium and rhodium together. Their surfaces adsorb the exhaust gases, weaken the bonds, and let three jobs happen at once: carbon monoxide is oxidized to CO2, unburnt fuel is oxidized to CO2 and water, and harmful nitrogen oxides are reduced back to harmless nitrogen. The metals are not consumed — they release the products and start again.
Pt, Pd and Rh in a catalytic converter turn CO, unburnt fuel and nitrogen oxides into harmless gases — without being used up.
'Noble' here means resistant to corrosion and oxidation, not chemically inert in every way — platinum still forms a rich array of complexes and is a powerful catalyst. Do not confuse the noble metals (corrosion-resistant transition metals) with the noble gases (the unreactive group-18 elements); the words are unrelated.