Coordinate (Dative Covalent) Bonds

What Is a Coordinate Bond?

A coordinate bond (also called a dative covalent bond) is a covalent bond in which both electrons in the shared pair come from the same atom. The atom that donates is called the donor (Lewis base); the atom that accepts is called the acceptor (Lewis acid).

Once formed, a coordinate bond is identical in strength and length to an ordinary covalent bond — the distinction is only in how it was formed.

Learning Goals: By the end of this guide, you should be able to:

Define a coordinate bond and distinguish it from a regular covalent bond.

Identify the donor (Lewis base) and acceptor (Lewis acid) in examples.

Draw coordinate bonds using the arrow notation (→).

Recognise coordinate bonds in $NH_4^+$ , $H_3O^+$ , $BF_3·NH_3$ , $[Cu(H_2O)_6]^{2+}$ , and CO.

How Coordinate Bonds Form

Requirements

Component	Role	Requirement
Donor (Lewis base)	Provides both electrons	Must have a lone pair
Acceptor (Lewis acid)	Receives the electron pair	Must have an empty orbital

Arrow Notation

Coordinate bonds are drawn with an arrow (→) pointing from the donor to the acceptor:

:NH_3 \rightarrow BF_3

The arrow shows that nitrogen donates its lone pair into boron's empty p orbital.

Classic Examples

1. Ammonium Ion ( $NH_4^+$ )

NH_3 + H^+ \rightarrow NH_4^+

Donor: $NH_3$ (lone pair on N)
Acceptor: $H^+$ (empty 1s orbital)
Result: 4 equivalent N-H bonds (3 ordinary + 1 coordinate, but all identical once formed)

2. Hydronium Ion ( $H_3O^+$ )

H_2O + H^+ \rightarrow H_3O^+

Donor: $H_2O$ (lone pair on O)
Acceptor: $H^+$ (empty orbital)

3. BF₃·NH₃ Adduct

BF_3 + :NH_3 \rightarrow F_3B \leftarrow NH_3

Boron has only 6 electrons (electron-deficient)
Nitrogen donates its lone pair into boron's empty 2p orbital
Boron achieves an octet; the adduct is much more stable

4. Carbon Monoxide (CO)

CO has a triple bond: one σ bond, one standard π bond, and one coordinate π bond where oxygen donates a lone pair to carbon. This gives carbon a lone pair, making CO an excellent ligand for transition metals.

5. Transition Metal Complexes

Cu^{2+} + 6H_2O \rightarrow [Cu(H_2O)_6]^{2+}

Each water molecule donates a lone pair from O into empty d/hybrid orbitals on $Cu^{2+}$ . These are six coordinate bonds, forming the hydrated copper(II) ion.

Complex	Ligand	Donor atom	Coordination number
$[Cu(H_2O)_6]^{2+}$	$H_2O$	O	6
$[Fe(CN)_6]^{4-}$	$CN^-$	C	6
$[Ag(NH_3)_2]^+$	$NH_3$	N	2
$[CuCl_4]^{2-}$	$Cl^-$	Cl	4

Worked Examples

Example 1: Identify the Coordinate Bond

Question: In $PCl_4^+$ , identify the coordinate bond.

Answer: Phosphorus normally forms 3 bonds in $PCl_3$ using 3 unpaired electrons. The fourth bond to $Cl^+$ is coordinate — P donates a lone pair to the empty orbital on $Cl^+$ .

Example 2: Why Does $BF_3$ Form an Adduct with $NH_3$ ?

$BF_3$ is electron-deficient (6 electrons on B). As a Lewis acid, it accepts the lone pair from $NH_3$ (Lewis base). The coordinate bond gives boron a full octet, stabilising the molecule.

Example 3: Aluminium Chloride Dimer

$AlCl_3$ dimerises to $Al_2Cl_6$ . Each monomer acts as both donor and acceptor: a Cl on one $AlCl_3$ donates a lone pair into the empty orbital of the Al on the other $AlCl_3$ , forming two bridging coordinate bonds.

Common Mistakes

Thinking coordinate bonds are weaker — Once formed, they are identical to ordinary covalent bonds. Bond strength and length are the same.
Drawing the arrow backwards — The arrow points FROM the donor TO the acceptor. $N \rightarrow B$ means nitrogen donates to boron.
Forgetting coordinate bonds in polyatomic ions — $NH_4^+$ and $H_3O^+$ both contain coordinate bonds. Examiners specifically ask about this.
Confusing with ionic bonds — A coordinate bond is still a shared pair of electrons. The donor doesn't lose the electrons — they are shared.

Exam Tips (A-Level / AP / IB)

Use the arrow notation (→) in structural formulas to show coordinate bonds.
In $NH_4^+$ , state: "All four N-H bonds are equivalent once formed — you cannot distinguish the coordinate bond experimentally."
Know the definition for full marks: "A covalent bond in which both electrons in the shared pair are donated by the same atom."
For transition metal complexes, name the ligand, donor atom, and coordination number.

Frequently Asked Questions

Can a coordinate bond be broken?

Yes, and when it breaks, both electrons return to the donor atom. For example, when $NH_4^+$ loses a proton, the lone pair returns to nitrogen.

Is a coordinate bond the same as a Lewis acid-base reaction?

Yes! Every coordinate bond formation is a Lewis acid-base interaction. The donor is the Lewis base; the acceptor is the Lewis acid.

Why is CO such a strong ligand?

CO can form coordinate bonds where C donates its lone pair to the metal (σ donation), and simultaneously accept electron density from metal d orbitals into its empty π* orbitals (π back-bonding). This synergistic bonding makes CO bind very strongly to transition metals.

Types of Chemical Bonds — Compare ionic, covalent, and coordinate bonding.
Orbital Hybridization — How hybrid orbitals provide the framework for bond formation.
Intermolecular Forces — Distinguished from coordinate bonds (intramolecular).

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