Lewis Acids and Bases

Introduction

Proton transfer is one form of acid-base chemistry, but inorganic compounds also react by sharing an electron pair with a centre that can accept it. BF₃ binds NH₃, Ag⁺ binds ammonia, and oxide ions add to nonmetal oxides. The Lewis definition names the electron-pair acceptor as acid and donor as base, letting one framework cover molecular adducts and coordination complexes.

Core explanation

A Lewis base must have an electron pair available for donation, such as the lone pair on NH₃ nitrogen or an O donor in H₂O. A Lewis acid must have a suitable accepting orbital or electron-deficient site, such as the six-electron boron centre in BF₃ or a metal cation. The generic event is A + :B → A←B, where the arrow indicates the pair originates on B. After formation, the resulting coordinate bond is a covalent interaction; it is not a permanently different class of bond simply because its electrons started on one partner.

BF₃ + NH₃ → F₃B←NH₃ is the standard molecular example. Boron in free BF₃ has three bonding domains and a trigonal-planar geometry. Nitrogen donates its lone pair, creating a four-coordinate boron environment in the adduct. The reaction does not transfer H⁺, so it is Lewis acid-base chemistry without a Brønsted–Lowry acid-base step. Formal charges in one Lewis depiction may place negative charge on B and positive on N; these are bookkeeping assignments, not proof of fully separated ions.

Metal coordination is the same broad idea. Ag⁺ + 2NH₃ ⇌ [Ag(NH₃)₂]⁺ forms a soluble complex. Silver is the electron-pair-accepting Lewis acid, and NH₃ ligands are bases. This equilibrium lowers free Ag⁺ and can dissolve AgCl(s): AgCl(s) + 2NH₃ ⇌ [Ag(NH₃)₂]⁺ + Cl⁻. The result links Lewis binding to a precipitation equilibrium. Cu²⁺ can similarly bind ammonia, producing a distinct colour after initial hydroxide chemistry.

Metal-aqua ions connect Lewis and Brønsted models. Al³⁺ accepts lone pairs from water to form a hydrated complex, a Lewis acid-base association. Once bound, a water ligand's O–H bond is polarised and can donate a proton to another water molecule, a Brønsted reaction. Using both labels is appropriate if each step is named; collapsing them into one vague “acid reaction” loses the mechanism.

The Lewis concept also applies to oxides. CO₂ or SO₃ can accept electron density from oxide O²⁻ to form an oxoanion framework, such as CO₂ + O²⁻ → CO₃²⁻. Carbon in CO₂ is the electron-pair-accepting centre in this formal description, while oxide is the donor. The complete bond redistribution involves more than one arrow in a detailed mechanism, but the Lewis classification is clear.

Lewis acidity is conditional: solvent molecules may already occupy the accepting centre, ligands may compete, and steric crowding can hinder binding. A bare ion's formal charge alone does not give an exact formation constant.

Step-by-step reasoning

1. Locate the available electron pair on the candidate Lewis base. 2. Locate a suitable accepting site on the acid. 3. Draw the electron-flow arrow from donor pair to acceptor. 4. Check charge and coordination number of the product. 5. Consider solvent, competing ligands and equilibrium before predicting extent.

Visual explanation

Draw :NH₃ with a lone-pair dot arrow toward the B centre of planar BF₃. Beneath it, draw two NH₃ molecules around Ag⁺ to give [Ag(NH₃)₂]⁺. Colour the donor-to-acceptor arrow consistently in both diagrams to show the common logic.

Real-world analogy

One partner brings a connector and the other offers an open socket. The connection forms only if the shapes and energies fit; a solvent or another ligand may already occupy the socket. Lewis acids and bases describe the donor–acceptor roles rather than guaranteeing every encounter reacts completely.

Real-world example

Complexing Ag⁺ with ammonia is part of the classical confirmation of chloride. When AgCl dissolves, chloride is still present in solution; the silver has moved into a soluble Lewis acid-base complex. This is why an apparently vanished precipitate can re-form when ligand availability changes.

Why?

Why can BF₃ accept a pair despite being a neutral molecule? Its boron centre has only six electrons in the straightforward Lewis description and an accessible accepting orbital. Overall molecular neutrality does not mean every atom is electronically saturated.

Common misconception

“Every Lewis acid-base reaction involves a proton” is false. BF₃–NH₃ adduct formation and Ag⁺–NH₃ coordination involve electron-pair donation without H⁺ transfer. Brønsted chemistry is a narrower subset of acid-base processes.

Worked example

In AgCl(s) + 2NH₃(aq) ⇌ [Ag(NH₃)₂]⁺(aq) + Cl⁻(aq), the two nitrogen lone pairs act as Lewis bases and Ag⁺ as Lewis acid. Complex formation removes free Ag⁺, shifting AgCl dissolution forward. The equation balances one silver, one chloride, two ammonia molecules and net zero charge on both sides.

Quick check

1. In BF₃ + NH₃, which atom supplies the new bonding electron pair? Answer: Nitrogen in NH₃ supplies its lone pair to boron in BF₃.

Exam focus

Identify donor and acceptor explicitly and draw the pair-flow arrow in the correct direction. Distinguish complex formation from proton transfer, and connect a ligand-binding equation to any observed solubility or colour change.

Advanced insight

Lewis acid-base bonding can be quantified by formation constants and understood with molecular orbitals. Hard/soft compatibility, donor orbital energy, acceptor orbital energy and steric factors all influence stability. The simple donor–acceptor label is an entry point to those deeper energetic questions.

Summary

Lewis bases donate electron pairs and Lewis acids accept them. BF₃–NH₃ adducts and metal ammine complexes exemplify the concept without proton transfer. Solvent and competing equilibria determine how far binding proceeds and whether a precipitate dissolves.

Practice questions

1. Which species is the Lewis acid in CO₂ + O²⁻ → CO₃²⁻? Answer: CO₂ accepts electron density at its carbon centre; oxide is the donor base. 2. Is NH₃ a Lewis base when it binds Ag⁺? Answer: Yes. Nitrogen donates a lone pair into the silver coordination sphere. 3. Why can Al³⁺ be a Lewis acid while its hydrated complex is a Brønsted acid? Answer: Al³⁺ accepts water lone pairs, then a coordinated water ligand can donate H⁺ to another base.