A coordination bond (also known as a dative covalent bond) forms when both electrons in the shared pair come from the same atom.
In the context of complex ions, this occurs when a ligand donates a lone pair of electrons to a transition metal cation.
Ligands can be ranked by field strength, from weak-field to strong-field: $$ \mathrm{I}^{-}<\mathrm{Br}^{-}<\mathrm{S}^{2-}<\mathrm{Cl}^{-}<\mathrm{F}^{-}<\mathrm{OH}^{-}<\mathrm{H}_2 \mathrm{O}<\mathrm{SCN}^{-}<\mathrm{NH}_3<\mathrm{CN}^{-} \approx \mathrm{CO} $$ and a stronger field gives a bigger split and a different colour.
Definition
Ligand
A molecule or ion that donates a lone pair of electrons to a central metal ion to form a coordination bond.
Example
Common ligands include water ($H_2O$), ammonia ($NH_3$), and chloride ions ($Cl^-$).
Other important definitions:
Transition Metal Cation: A positively charged ion of a transition metal, which has vacant orbitals capable of accepting electron pairs from ligands.
Complex Ion: A charged species consisting of a central metal ion surrounded by ligands bonded through coordination bonds.
Tip
Ligands must have at least one lone pair of electrons to form a coordination bond with the metal ion.
How Are Coordination Bonds Formed?
Coordination bonds are formed when ligands donate their lone pairs of electrons to the empty orbitals of a transition metal cation.
This process can be broken down into the following steps:
Electron Pair Donation: The ligand, which has a lone pair of electrons, approaches the transition metal cation.
Orbital Overlap: The lone pair of electrons from the ligand overlaps with an empty orbital on the metal ion.
Bond Formation: A coordination bond is established, resulting in the formation of a complex ion.
Why Transition Metals?
Transition metals are particularly suited to forming complex ions because:
They have vacant d-orbitals that can accept electron pairs.
Their high charge density attracts electron-rich ligands.
Let’s examine the formation of the tetraamminecopper(II) ion, $[Cu(NH_3)_4]^{2+}$, as an example of a complex ion.
The Metal Ion:
Copper(II) ion ($Cu^{2+}$) acts as the central metal ion.
It has a high charge density and vacant orbitals to accept electron pairs.
The Ligand:
Ammonia ($NH_3$) molecules act as ligands.
Each $NH_3$ molecule has a lone pair of electrons on the nitrogen atom.
Bond Formation:
Four $NH_3$ molecules donate their lone pairs to the $Cu^{2+}$ ion, forming four coordination bonds.
This results in the complex ion $[Cu(NH_3)_4]^{2+}$.
The structure of $[Cu(NH_3)_4]^{2+}$ can be represented as:
$$ [Cu(NH_3)_4]^{2+} $$
Here:
The central $Cu^{2+}$ ion is surrounded by four $NH_3$ ligands.
The overall charge of the complex ion is $+2$, which is the sum of the charge on the metal ion and the neutral ligands.
Note
In the $[Cu(NH_3)_4]^{2+}$ complex, the copper ion accepts four lone pairs of electrons from four ammonia molecules, forming a stable coordination complex.
Determining the Charge on a Complex Ion
To deduce the charge on a complex ion, you can use the oxidation state of the central metal ion and the charges of the ligands present.
Ligands can be either neutral (e.g., $H_2O$, $NH_3$) or negatively charged (e.g., $Cl^-$, $CN^-$).
The overall charge of the complex ion is the sum of the oxidation state of the metal and the charges contributed by all ligands.
Example
In the complex ion $[\text{Fe(CN)}_6]^{4-}$:
Cyanide ($CN^-$) carries a charge of -1 each, contributing a total charge of $-6$ ($6 \times -1$).
The overall charge of the complex ion is -4, so the oxidation state of iron must be +2 since $+2 + (-6) = -4$.
Tip
By carefully summing the metal oxidation state and ligand charges, you can systematically determine the charge of any complex ion.
Common ions formed by the transition elements:
Properties of Complex Ions
Complex ions exhibit unique properties due to the interaction between the metal ion and the ligands. These include:
Color: Many complex ions are colored due to electronic transitions within the d-orbitals of the metal ion.
Magnetism: The magnetic properties of a complex ion depend on the number of unpaired electrons in the metal ion.
Stability: The stability of a complex ion depends on factors such as the charge density of the metal ion and the nature of the ligands.
Tip
The color of a complex ion can often be used to identify the metal ion and its oxidation state.
Common Mistake
Do not confuse coordination bonds with ionic bonds, and remember that a coordination bond is a type of covalent bond.
Coordination bonds involve the donation of a lone pair from a ligand to a metal ion, whereas covalent bonds involve the sharing of electrons between two atoms.
You can recap the concept of a coordination bond in the article Structure 2.2.3.
Active recall
What is the role of a ligand in the formation of a coordination bond?
Why are transition metals particularly suited to forming complex ions?
