Coordinate Bonding and Donor–Acceptor Pairs

Introduction

Some covalent bonds form when one species supplies both electrons of a new shared pair. Ammonia's nitrogen lone pair can bind H⁺ to form NH₄⁺. “Coordinate” describes how the bond formed; it does not mean the final bond is held together by a different force from other covalent bonds.

Core explanation

NH₃ has three N–H bonds and one lone pair on nitrogen in a simple Lewis diagram. A proton H⁺ has no electron. Nitrogen donates its lone pair to form a fourth N–H bond: NH₃ + H⁺ → NH₄⁺. In a formation diagram, an arrow from N's lone pair toward H⁺ shows the electron-pair donation. Nitrogen is the Lewis base, and H⁺ is the Lewis acid. The product has overall +1 charge, consistent with the reactants.

Once ammonium forms, its four N–H bonds are equivalent in the usual tetrahedral structural description. A label identifying one as “the coordinate bond” remembers its formation history, not a persistent special physical marker on one N–H bond. The electrons and nuclei in the ion do not retain a tag telling which hydrogen arrived last. This distinction prevents over-literal reading of a dotted-arrow drawing.

A neutral electron-pair acceptor can also form an adduct. BF₃ is electron deficient at boron in a simple Lewis model. NH₃ can donate its lone pair to BF₃, forming an F₃B←NH₃ adduct (arrow convention may be written donor-to-acceptor). The N–B bond is formed from nitrogen's pair, and formal-charge bookkeeping may assign N positive and B negative in a fully covalent Lewis representation. The net adduct remains neutral. Its geometry around B changes as a fourth bond forms.

Coordinate bonding is one application of the broader Lewis acid-base definition: a base gives an electron pair, and an acid accepts it. This differs from the narrower Brønsted acid-base description focused on proton transfer, although NH₃ + H⁺ fits both descriptions. BF₃ + NH₃ contains no proton transfer, so the Lewis concept is especially useful.

Not every lone-pair donor reacts equally strongly with every acceptor. Solvent, competing ligands, steric access and orbital energy matter. An arrow on paper identifies a plausible donation pattern under suitable conditions, not an unconditional yield prediction.

Step-by-step reasoning

1. Draw the donor's Lewis structure and locate an available lone pair. 2. Identify an acceptor with an electron-deficient site or empty suitable orbital. 3. Draw an arrow from donor pair to acceptor. 4. Form a new covalent bond and audit formal charges and total charge. 5. Describe the final structure without treating the new bond as permanently unique.

Visual explanation

Draw NH₃ as a pyramid with a lone pair above N, then H⁺ approaching along that direction. On the right draw tetrahedral NH₄⁺ with four equivalent N–H lines and brackets carrying +. A second panel shows the N lone pair pointing toward B in BF₃.

Real-world analogy

Two people can pay for a shared meal even if one person initially supplies all the money. Once the meal is shared, the origin of payment does not create a different kind of meal. Electron-pair origin describes bond formation, not a unique final bond type.

Real-world example

Ammonium ions form when ammonia encounters acids. Their presence in many salts is explained by nitrogen's available lone pair accepting a proton, with the resulting NH₄⁺ behaving as a polyatomic cation in ionic compounds.

Why?

Why can BF₃ accept a pair? Boron has an incomplete octet in the simple BF₃ Lewis picture and can accommodate an electron pair from a suitable donor. The adduct's bond formation changes electron distribution and local geometry.

Common misconception

“The coordinate N–H bond in NH₄⁺ is weaker or visibly different from the other three.” In the ordinary ammonium ion, all four N–H bonds are equivalent; the coordinate label records the route of formation.

Worked example

For NH₃ + H⁺ → NH₄⁺, count valence electrons in the product: N supplies 5, four H atoms supply 4 and the + charge removes 1, giving 8 valence electrons. Four N–H bonds use all 8. There are no lone pairs on N in the simple NH₄⁺ Lewis diagram. Nitrogen has four bonds and formal charge +1; each H has formal charge 0. This agrees with the ion's net charge.

Quick check

1. Which partner is the electron-pair donor in NH₃ + BF₃? Answer: NH₃, through nitrogen's lone pair.

Exam focus

Draw the arrow from donor to acceptor, not the reverse. Audit overall charge after bond formation. State that a coordinate bond is covalent and that equivalent bonds in a final ion need not remain distinguishable.

Advanced insight

Coordination complexes extend this donor–acceptor idea: ligands donate electron pairs to a metal centre. Detailed metal–ligand bonding can involve orbital mixing and back-donation, so a simple arrow is only the starting description.

Summary

A coordinate bond forms when one partner supplies a shared electron pair. NH₃ donates to H⁺ or BF₃ as a Lewis base. The donor–acceptor arrow explains formation, while the final covalent bond may be equivalent to other bonds in the product.

Practice questions

1. Identify Lewis acid and base in NH₃ + H⁺. Answer: NH₃ is the electron-pair-donating base; H⁺ is the accepting acid. 2. How many N–H bonds are in NH₄⁺? Answer: Four. 3. Are the four N–H bonds in ordinary NH₄⁺ distinguishable by their formation history? Answer: No. They are equivalent in the final ion's standard structure. 4. Why is BF₃ a Lewis acid in its adduct with NH₃? Answer: Boron accepts nitrogen's lone pair to make a new bond.