In the vast spectrum of chemical bonding, coordination covalent bonding stands out with unique characteristics and applications. Delve into the intriguing world of coordination compounds and the bonds that define them.
What Constitutes a Coordination Bond?
A coordination bond, also known as a dative covalent bond, is a special type of covalent bond wherein both the shared electrons originate from only one of the atoms. This contrasts with typical covalent bonds where each atom provides one electron.
- Donor Atom: The atom that donates both electrons is often called the ‘donor’ atom. This atom usually has a lone pair of electrons which it can donate.
- Acceptor Atom: The atom that accepts the electron pair is called the ‘acceptor’ atom or sometimes referred to as the ‘Lewis acid’.
Image courtesy of Reuel Sa
Criteria for Identifying Coordination Bonds
Coordination bonds are prevalent in many compounds, especially in coordination complexes. Here are some criteria to identify them:
- Presence of a Lone Pair: Look for atoms or ions with a lone pair of electrons. Common donor atoms include nitrogen, oxygen, and chlorine.
- Metal Centres: Coordination bonds are often formed with transition metals, which act as acceptors. These metals can form coordination complexes by accepting lone pairs from donor atoms or ligands.
- Charge Distribution: Though the bond is covalent, the metal centre may possess a positive formal charge while the ligand may carry a negative charge, signifying the electron donation.
A coordinate covalent bond- An arrow with its head pointing toward the electron acceptor and its tail pointing toward the donor
Image courtesy of Nonagonal Spider
Coordination Bonds in Lewis Acid-Base Reactions
The concept of Lewis acids and bases further elucidates the nature of coordination bonds:
- Lewis Acid: A species that can accept an electron pair. In the context of coordination bonding, transition metals and other electron-deficient species act as Lewis acids.
- Lewis Base: A species that can donate an electron pair. Ligands in coordination compounds, with their lone pairs, act as Lewis bases.
An example of this is the reaction between boron trifluoride (BF₃) and ammonia (NH₃). BF₃ acts as a Lewis acid, accepting a lone pair from NH₃, which acts as a Lewis base. This results in the formation of a coordination bond, giving the complex ion [F₃B←NH₃].
Image courtesy of OpenStax
Transition Element Complexes in Coordination Bonding (Higher Level)
Transition elements hold a unique position in the realm of coordination chemistry due to their ability to form a variety of coordination compounds.
- D-orbitals: Transition metals possess vacant d-orbitals, which can accept electron pairs from ligands, making them ideal centres for coordination complexes.
- Variable Oxidation States: Transition metals can exist in various oxidation states. This ability allows them to form coordination compounds with different numbers of ligands and varied geometries.
- Colourful Complexes: Many transition metal complexes are coloured. The colour arises due to the absorption of specific wavelengths of light, leading to electronic transitions within the d-orbitals.
- Coordination Number: The number of coordination bonds formed by the metal atom in the complex is its coordination number. Common coordination numbers include 4 (tetrahedral or square planar) and 6 (octahedral).
For instance, the hexaaquacopper(II) ion, [Cu(H₂O)₆]²⁺, is a coordination complex of copper in the +2 oxidation state. Here, copper acts as the Lewis acid, accepting electron pairs from six water molecules, which act as the Lewis bases.
![Hexaaquacopper(II) ion, [Cu(H₂O)₆]²⁺, a coordination complex of copper in the +2 oxidation state.](https://tutorchase-resources-platform-client-production.vercel.app/_next/image?url=https%3A%2F%2Ftutorchase-production.s3.eu-west-2.amazonaws.com%2F80879228-834d-467e-8bf0-bf99cf03b7e1-file.png&w=1920&q=75)
Image courtesy of breakingatom.
In summary, coordination covalent bonding adds depth to our understanding of chemical bonding, showcasing the versatility and intricacies of atom-to-atom interactions. Its significance, especially in the context of transition metal chemistry, underscores the richness and diversity of the chemical landscape.
FAQ
Coordination compounds find a wide array of applications in the real world due to their unique properties. For instance, they are pivotal in biological systems, with haemoglobin (binding oxygen via an iron centre) being a key example. Moreover, they serve vital roles in industrial catalysts, improving reaction efficiency. Certain coordination compounds also act as complexing agents in water softening processes. Furthermore, they have been utilised in cancer treatments and are at the heart of vibrant colour displays in many fireworks.
Chelating ligands are a special subset of ligands that can form more than one coordination bond with the central metal atom/ion due to the presence of multiple donating sites. This enables them to effectively 'hug' or 'encircle' the metal atom/ion, reminiscent of a claw (the term "chelate" originates from the Greek word for claw). Ethylenediaminetetraacetic acid (EDTA) is a prominent example of a chelating ligand that can form up to six coordination bonds with a central metal ion.
The geometry or shape of a coordination compound primarily hinges on the central metal atom/ion's coordination number, which refers to the number of ligand atoms or groups bonded to it. Common coordination numbers include 2 (linear), 4 (tetrahedral or square planar), and 6 (octahedral). The electronic configuration of the central atom/ion, the type of ligands attached, and their spatial requirements all play roles in defining the final geometry of the coordination compound. It's worth noting that some transition metals can adopt multiple geometries based on specific conditions and ligands involved.
A ligand is an ion or molecule that can donate a pair of electrons to a central metal atom or ion to form a coordination bond. Ligands possess at least one lone pair of electrons that they use to form the dative bond with the central atom/ion in a coordination compound. Examples include H₂O, NH₃, CN⁻, and many others. The number of ligand atoms or groups that can attach to the central metal atom/ion defines the coordination number of the compound.
Transition metals are particularly adept at forming coordination compounds due to their unique electronic configuration, which includes partially filled d-orbitals. These d-orbitals can accommodate lone pairs of electrons from ligands. Moreover, transition metals exhibit a wide range of oxidation states, providing flexibility in bond formation with various ligands. This leads to a variety of coordination compounds with different geometries, charges, and properties, making transition metals central players in the realm of coordination chemistry.
Practice Questions
A coordination bond, also referred to as a dative covalent bond, is a unique type of covalent bond in which both electrons involved in the bond come from only one of the two atoms. This contrasts with a typical covalent bond where each atom contributes one electron. The atom donating both electrons is termed the 'donor', typically possessing a lone pair, whereas the receiving atom is the 'acceptor'. An illustrative example of a molecule containing a coordination bond is the complex ion formed between boron trifluoride (BF₃) and ammonia (NH₃), denoted as [F₃B←NH₃].
Transition elements play a pivotal role in coordination bonding, primarily due to their d-orbitals and variable oxidation states. These metals often have vacant d-orbitals, which can accept electron pairs from ligands, facilitating the formation of coordination complexes. Their ability to exist in a variety of oxidation states adds another layer of versatility, allowing them to bond with different ligands, leading to coordination compounds with varied geometries and coordination numbers. For instance, the hexaaquacopper(II) ion, [Cu(H₂O)₆]²⁺, showcases copper in the +2 oxidation state, forming coordination bonds with six water molecules, aided by its d-orbitals and specific oxidation state.
