The familiar definition of an acid, something that donates a hydrogen ion, works well for reactions like vinegar reacting with baking soda, but it genuinely breaks down for a great deal of inorganic chemistry involving metal ions and other species that have no hydrogen ion to give up at all. Inorganic acid-base chemistry instead extends to the broader Lewis definition: an acid is anything that accepts an electron pair, and a base is anything that donates one, a definition that lets metal ions and many other species with no hydrogen ion available still count as genuine acids.
The Lewis definition looks at electron pairs rather than hydrogen ions specifically
Under the Lewis definition, an acid-base reaction is fundamentally about one species with an available empty spot for electrons, an acid, accepting a donated electron pair from another species that has one to spare, a base, forming a new bond between them in the process. This reframing captures the same essential logic as the familiar hydrogen-ion definition for reactions that do involve hydrogen ions, but it also extends cleanly to reactions that clearly behave like acid-base chemistry, forming exactly the same kind of new bond, without any hydrogen ion being involved at all.
Metal ions frequently act as Lewis acids by accepting electron pairs from surrounding ligands
A metal ion typically carries a positive charge and an available empty orbital capable of accepting a donated electron pair, which is exactly what happens when a metal ion forms a coordination compound with surrounding ligands: each ligand donates an electron pair to the metal, behaving as a Lewis base, while the metal accepts that pair, behaving as a genuine Lewis acid, even though no hydrogen ion is anywhere in the picture. This is precisely why inorganic chemistry treats metal-ligand bonding as a specific instance of Lewis acid-base chemistry, using the same underlying electron-pair logic that governs the more familiar hydrogen-ion acid-base reactions studied elsewhere.
What we're still unsure about
That the Lewis definition correctly generalises acid-base chemistry to electron-pair transfer, covering metal-ion reactions the hydrogen-ion definition can't explain, is well established, extensively confirmed inorganic chemistry taught consistently since the definition was first proposed in the 1920s. What's more genuinely a matter of ongoing predictive nuance is exactly how strongly a given Lewis acid and Lewis base will actually pair up preferentially when several possible combinations are available, since some Lewis acid-base pairings form more readily than others based on more subtle electronic and structural compatibility factors, and chemists continue to refine predictive frameworks for exactly which Lewis acid-base pairs bond most favourably in a given real chemical system.
This sits inside Acid-Base Chemistry in Inorganic Systems, one of seven topics in Inorganic Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.