Interstitial Compounds and Compound-Existence Rules

Interstitial compounds form when small atoms (H, C, N, or B) occupy the empty spaces (interstitial sites) within a metal crystal lattice, without fundamentally disrupting the metallic bonding network. Because the underlying metal lattice and its delocalised bonding are largely preserved, interstitial compounds RETAIN metallic conductivity (electrons can still move freely through the metal framework), and the small interstitial atoms typically increase hardness and raise the melting point relative to the pure metal (they restrict how easily metal layers can slide past each other). Chemically, interstitial compounds tend to be UNREACTIVE (chemically inert), not reactive — the small non-metal atoms are tucked away inside the lattice, largely inaccessible to react with anything outside. Chemically very reactive is therefore the one property that does NOT describe interstitial compounds; the other three (high melting point, hardness, retained metallic conductivity) all do.

Only a specific subset of metal carbides count as true interstitial carbides: those formed by larger transition metals (like tungsten, forming WC, or titanium, forming TiC) whose atomic radius is large enough that carbon atoms fit into the octahedral holes of the metal lattice without significantly distorting it, preserving metallic character. Smaller or more electropositive metals instead form very different carbide types: silicon carbide (SiC) is a covalent network solid (not interstitial and not really a classic carbide in the ionic sense at all), calcium carbide (CaC₂) is an ionic, salt-like carbide containing discrete C2²- (acetylide) ions, and beryllium carbide (Be₂C) is similarly an ionic, salt-like carbide containing discrete C⁴⁻ (methanide) ions. Only WC-type carbides, from large enough transition metals, are genuinely interstitial.

CuI₂ does not exist because Cu²⁺ is a strong enough oxidizing agent to oxidize iodide ion before a stable Cu₂+/I⁻ salt can form: Cu²⁺ + 2I- would instead react as 2Cu₂⁺ + 4I- → 2CuI (copper(I) iodide, a stable solid) + I₂, with copper being reduced to +1 and iodide being oxidized to iodine, rather than remaining as a simple Cu(II) diiodide salt. This is the same underlying idea as Fe³⁺ oxidizing iodide (covered in the companion cluster on redox trends): whenever a metal ion oxidation state is thermodynamically capable of oxidizing a particular counter-ion, a simple salt pairing that cation directly with that anion will not exist — the redox reaction happens instead, giving different products.

Several important biomolecules are built around a metal ion at their functional core: haemoglobin uses iron (Fe, a transition metal, at the centre of its heme group, responsible for oxygen binding), vitamin B₁₂ uses cobalt (Co, a transition metal, at the centre of its corrin ring), and cisplatin is a coordination compound built directly around platinum (Pt, a transition metal). Chlorophyll, by contrast, is built around magnesium (Mg) at the centre of its porphyrin-like ring — and magnesium is an ALKALINE EARTH METAL (group 2, s-block), not a transition metal at all. Whenever a question asks which of these classic biomolecule examples does not contain a transition metal, chlorophyll (with its Mg centre) is the one to pick.