Which one of the following contains all Lewis acids?

Correct — A, AlCl3, Co3+, BF3. On the definition G. N. Lewis put forward in 1923, an acid is anything that accepts a pair of electrons and a base is anything that donates one; the two join to give an adduct, with the new bond made entirely from the base's pair. Two kinds of species accept. The first is a molecule whose central atom has not completed its octet: boron in BF₃ has only six electrons around it and an empty orbital waiting, and aluminium in AlCl₃ is in the same position, which is exactly why both are the standard catalysts for Friedel–Crafts reactions. The second is a small, highly charged cation, which pulls electron density towards itself and has vacant orbitals to receive it — Co³⁺ is a textbook example, and every cobalt(III) complex, [Co(NH₃)₆]³⁺ among them, is a Lewis adduct with the metal ion as the acid and the six ammonia molecules as bases. So all three members of set (a) accept, and the set qualifies. Each of the other three sets is spoiled by at least one donor. The quick way to scan them is to look for a lone pair that is free to be given away: nitrogen in NH₃ has one, oxygen in H₂O has two, and OH⁻ and F⁻ are anions loaded with them. Any species carrying an available lone pair is a base in this scheme, and one base in a set is enough to disqualify it.

• (b)BCl3, NH3, OH⁻ — BCl₃ is a genuine Lewis acid, boron again being short of an octet. But NH₃ donates the lone pair on nitrogen and OH⁻ is an anion rich in them; both are bases.
• (c)Mg2+, H2O, F⁻ — Mg²⁺ accepts, as a small doubly charged cation should. Water and the fluoride ion do the opposite — both carry lone pairs they can give away, so two of the three are bases.
• (d)BF3, NH3, H2O — The classic acid with the classic bases. BF₃ accepts; NH₃ and H₂O donate. F₃B←NH₃ is the adduct every textbook draws to introduce the definition, so this set is one acid and two bases.

There are three definitions of acids and bases in ordinary use and they widen in turn. Arrhenius requires an acid to release H⁺ in water and a base to release OH⁻. Brønsted and Lowry replace that with proton transfer, so an acid is a proton donor and a base a proton acceptor, which frees the idea from water. Lewis, in the same year of 1923, goes wider still and drops the proton altogether: an acid accepts an electron pair, a base donates one. That is why the Lewis scheme can call BF₃ an acid though it has no hydrogen at all, and why it covers the whole of coordination chemistry, in which a metal ion is the acid and the ligands are the bases.

The efficient way to attack this item is to hunt for the bases rather than to verify the acids. Lone pairs are visible at a glance — nitrogen in ammonia, oxygen in water, and any anion such as hydroxide or fluoride — so a single pass across the four options strikes out (b), (c) and (d) without any analysis of the acids at all. It is also worth being clear about what the definitions have in common. A Brønsted acid is a subset of the Lewis picture rather than a rival to it, since a proton is the smallest possible electron-pair acceptor. What the Lewis definition adds is everything with an empty orbital and no hydrogen: boron and aluminium halides, metal cations, and molecules whose octet can expand, such as SiF₄ and SnCl₄.

• G. N. Lewis proposed the electron-pair definition in 1923, the same year that Brønsted and Lowry proposed the proton-transfer definition.
• A Lewis acid accepts an electron pair; a Lewis base donates one; the product is called an adduct.
• Molecules with an incomplete octet — BF₃, BCl₃, AlCl₃ — are Lewis acids, and are used as Friedel–Crafts catalysts for that reason.
• Small, highly charged metal cations such as Co³⁺, Mg²⁺, Al³⁺ and Fe³⁺ act as Lewis acids.
• Species carrying an available lone pair — NH₃, H₂O, OH⁻, F⁻, CN⁻ — are Lewis bases.
• Every complex ion is a Lewis adduct: in [Co(NH₃)₆]³⁺ the cobalt(III) ion is the acid and the six ammonia molecules are the bases.

• Treating water as a Lewis acid because it is amphoteric in the Brønsted sense; as a Lewis species it donates.
• Assuming an acid must contain hydrogen — BF₃ and AlCl₃ have none.
• Overlooking that an anion such as OH⁻ or F⁻ is almost always a base in this scheme.

As a set-identification item like this one, as a single 'which is a Lewis acid' question, or through coordination chemistry, where the metal ion and the ligands have to be labelled acid and base.

• practice — not a real PYQ

  In the reaction BF₃ + NH₃ → F₃B–NH₃, the species acting as the Lewis base is

  • (a)BF₃
  • (b)NH₃
  • (c)both
  • (d)neither

  Answer(b) NH₃ — nitrogen donates its lone pair to the empty orbital on boron, so ammonia is the base and BF₃ the acid.

• practice — not a real PYQ

  In the complex ion [Cu(NH₃)₄]²⁺, the Lewis acid is

  • (a)NH₃
  • (b)Cu²⁺
  • (c)the whole ion
  • (d)there is no Lewis acid present

  Answer(b) Cu²⁺ — the metal ion accepts electron pairs from the four ammonia ligands, which are the bases.