Coordinate Covalent (Dative) Bonds

A coordinate covalent bond (dative bond) is a covalent bond in which both shared electrons come from the same atom, called the donor, which has a lone pair, while the other atom, the acceptor, has an empty orbital. In brief: in the ammonium ion, NH₄⁺, ammonia donates its lone pair to a hydrogen ion (H⁺), which has no electrons, and once formed, the new N–H bond is identical to the three other N–H bonds.

Dative bonds are found in many ions and molecules, from the hydronium ion in water to the complexes of metals with ligands and the binding of oxygen to hemoglobin. They are also the key to the Lewis definition of acids and bases. This article explains how dative bonds form, how to draw them, lists common examples in a table, describes complex ions, works through examples of counting dative bonds and charges, and gives some history.

What is a coordinate covalent bond?

In an ordinary covalent bond, each atom contributes one electron to the shared pair. In a coordinate covalent bond, one atom supplies both electrons. The atom that provides the pair is the electron-pair donor (a Lewis base), and it must have a lone pair. The atom that receives the pair is the acceptor (a Lewis acid), and it must have an empty orbital. After the bond is formed, it cannot be distinguished from a normal covalent bond, since the electrons in the bond are identical. Only the origin of the electrons is different.

A dative bond is often drawn with an arrow pointing from the donor to the acceptor (D → A), or as a normal line with the formal charges shown (D⁺–A⁻).

What are some examples?

• Ammonium ion, NH₄⁺: NH₃ + H⁺ → NH₄⁺. The lone pair on nitrogen forms the bond to H⁺. All four N–H bonds are equal.
• Hydronium ion, H₃O⁺: H₂O + H⁺ → H₃O⁺. This is the form of the hydrogen ion in water.
• Boron trifluoride-ammonia adduct, F₃B←NH₃: boron in BF₃ has an empty p orbital and accepts the lone pair from ammonia to complete its octet.
• Carbon monoxide, CO: the structure C≡O has two normal covalent bonds and one dative bond from oxygen to carbon, which explains the unusual bonding of the molecule with a triple bond.
• Dimer of aluminum chloride, Al₂Cl₆: two bridging chlorine atoms each donate a lone pair to an aluminum atom of the other half.
• Complex ions: [Cu(NH₃)₄]²⁺, [Fe(CN)₆]⁴⁻ and hemoglobin with oxygen. See below.

Donors, acceptors and the species formed

What are complex ions?

A complex ion (or coordination complex) has a central metal ion bonded to a number of molecules or ions called ligands by dative bonds. The ligands donate lone pairs to empty orbitals of the metal. The number of bonds between the metal and its ligands is the coordination number, commonly 2, 4 or 6. Transition metal ions are especially good acceptors, since they have empty d, s and p orbitals of suitable energy. The charge on the complex is the sum of the metal’s charge and the charges of the ligands. Complexes are often colored: aqueous copper(II) ions are pale blue, and the ammonia complex is deep blue.

Common ligands

Worked examples

1. Formal charges in NH₄⁺. Nitrogen has 4 bonds and no lone pair: 5 − 0 − 4 = +1. Each H: 0. The sum is +1. The formal charge of +1 on nitrogen shows where the donated lone pair is now shared.
2. Dative bonds in [Cu(NH₃)₄]²⁺. There are 4 N→Cu bonds, one from each ammonia ligand. The charge of the complex is the charge of Cu²⁺ plus 4 × 0 = 2+.
3. Charge of [Fe(CN)₆]³⁻. Iron is in the +3 state and each of six cyanide ligands has a charge of −1. The total is +3 + 6(−1) = −3. The ion is hexacyanidoferrate(III), with a coordination number of 6.
4. Oxidation number of cobalt in [Co(NH₃)₆]Cl₃. The three chloride ions outside the complex have a total of −3, so the complex ion has a charge of +3. With neutral ammonia ligands, the cobalt is +3. The name is hexaamminecobalt(III) chloride.
5. Which species can be a donor? NH₃ (lone pair on N): yes. CH₄ (no lone pair): no. BF₃ (empty orbital on B, no lone pair on B): acceptor. H₂O: donor.
6. Moles in a reaction. If 0.050 mol of copper(II) sulfate reacts with excess ammonia, 0.050 mol of [Cu(NH₃)₄]²⁺ is formed, using 4 × 0.050 = 0.20 mol of NH₃. The mass of ammonia (17.03 g/mol) is 0.20 × 17.03 = 3.4 g.

How are dative bonds related to Lewis acids and bases?

In 1923, G. N. Lewis defined an acid as an electron-pair acceptor and a base as an electron-pair donor. The formation of a coordinate covalent bond is an acid-base reaction in the Lewis sense: the base donates its lone pair to the acid. This definition is wider than the Brønsted-Lowry definition, since it includes reactions without hydrogen ions such as BF₃ + NH₃ and the formation of metal complexes. The proton transfer reaction NH₃ + H⁺ → NH₄⁺ fits both definitions.

Who developed the concept?

The Swiss chemist Alfred Werner proposed the theory of coordination compounds in 1893, with the idea of a central metal ion surrounded by ligands, and received the Nobel Prize in Chemistry in 1913. Nevil Sidgwick introduced the term “coordinate link” in the 1920s and applied the electronic theory of valence to complexes. Lewis’s definition of acids and bases dates from 1923. The role of dative bonds in biology became clear in the 20th century with the structure of hemoglobin and chlorophyll, which both contain a metal ion with ligands.

Which mistakes are common?

Mistake 1: Thinking that a dative bond is weaker or different

Once formed, it is a normal covalent bond. In NH₄⁺ all four N–H bonds are identical.

Mistake 2: Forgetting that the donor needs a lone pair

A donor must have a nonbonding pair of electrons.

Mistake 3: Mixing up the direction of the arrow

The arrow points from the donor to the acceptor.

Mistake 4: Ignoring the ligand charges when finding the metal’s oxidation state

Add the charges of the ligands and subtract from the charge of the complex to find the oxidation state of the metal.