A covalent coordinate bond or coordination bond is a type of bond in which one of the attached atoms supplies all of the shared electrons.
In a simple covalent bond, each atom supplies one electron to the bond. On the other hand, in a coordination bond, the atoms that donate the electron to form a bond are called the donor atom, while the atom that accepts the pair of electrons to join is called the acceptor atom (Clark, 2012).
Figure 1: representation of a coordination bond between the donor atom (N) and the acceptor (H).
A coordination bond is represented by an arrow that starts from the donor atoms and ends at the acceptor atom (Figure 1). In some cases, the donor can be a molecule.
In this case, an atom in the molecule can donate the electron pair, which would be the Lewis base, while the molecule with the accepting capacity would be the Lewis acid (Coordinate Covalent Bond, SF).
A coordination bond has characteristics similar to that of a simple covalent bond. Compounds that have this type of bond generally have a low melting and boiling point, with a non-existent coulombic interaction between the atoms (unlike the ionic bond), and the compounds are very soluble in water (Atkins, 2017).
Some examples of coordinate covalent bonds
The most common example of a coordination bond is the ammonium ion, which is formed by the combination of an ammonia molecule and a proton from an acid.
In ammonia, the nitrogen atom has a lone pair of electrons after completing its octet. Donate this lone pair to the hydrogen ion, thus the nitrogen atom becomes a donor. The hydrogen atom becomes the acceptor (Schiller, SF).
Figure 2: representation of the hydronium ion coordination bond.
Another common example of a dative bond is the formation of the hydronium ion. As with the ammonium ion, the free electron pair of the water molecule serves as a donor to the proton that is the acceptor (Figure 2).
However, it should be noted that once the coordination bond has been established, all hydrogens attached to oxygen are exactly equivalent. When a hydrogen ion breaks down again, there is no discrimination between which of the hydrogens is released.
An excellent example of a Lewis acid-base reaction, illustrating the formation of a coordinate covalent bond, is the boron trifluoride adduct formation reaction with ammonia.
Boron trifluoride is a compound that does not have a noble gas structure around the boron atom. Boron only has 3 pairs of electrons in its valence shell so BF3 is said to be electron deficient.
The unshared electron pair of ammonia nitrogen can be used to overcome that deficiency, and a compound is formed that involves a coordination bond.
Figure 3: Adduct between boron trifluoride molecule and ammonia.
That pair of electrons from nitrogen is donated to boron's empty p orbital. Here ammonia is the Lewis base and BF3 is the Lewis acid.
Coordination chemistry
There is a branch of inorganic chemistry dedicated exclusively to the study of compounds that form transition metals. These metals join other atoms or molecules through coordination bonds to form complex molecules.
These molecules are known as coordination compounds and the science that studies them is called coordination chemistry.
In this case, the substance attached to the metal, which would be the electron donor, is known as a ligand and coordination compounds are commonly known as complexes.
Coordination compounds include substances such as vitamin B12, hemoglobin and chlorophyll, dyes and pigments, and catalysts used in the preparation of organic substances (Jack Halpern, 2014).
An example of a complex ion would be the cobalt 2+ complex, which would be dichloroaminenethylenediamine cobalt (IV).
Coordination chemistry grew out of the work of Alfred Werner, a Swiss chemist who examined various compounds of cobalt (III) chloride and ammonia. After the addition of hydrochloric acid, Werner found that the ammonia could not be completely removed. He then proposed that the ammonia should be more closely bound to the central cobalt ion.
However, when aqueous silver nitrate was added, one of the products formed was solid silver chloride. The amount of silver chloride formed was related to the number of ammonia molecules bound to cobalt (III) chloride.
For example, when silver nitrate was added to CoCl 3 · 6NH 3, all three chlorides were converted to silver chloride.
However, when silver nitrate was added to CoCl 3 · 5NH 3, only 2 of the 3 chlorides formed silver chloride. When CoCl 3.4NH 3 was treated with silver nitrate, one of the three chlorides precipitated as silver chloride.
The resulting observations suggested the formation of complex or coordination compounds. In the internal coordination sphere, also referred to in some texts as the first sphere, the ligands are directly attached to the central metal.
In the outer sphere of coordination, sometimes called the second sphere, other ions are attached to the complex ion. Werner was awarded the Nobel Prize in 1913 for his theory of coordination (Introduction to Coordination Chemistry, 2017).
This theory of coordination makes transition metals have two types of valence: the first valence, determined by the oxidation number of the metal, and the other valence called the coordination number.
The oxidation number tells how many covalent bonds can be formed in the metal (example iron (II) produces FeO) and the coordination number tells how many coordination bonds can be formed in the complex (example iron with coordination number 4 produces - and 2-) (Coordination Compounds, 2017).
In the case of cobalt, it has a coordination number 6. That is why in Werner's experiments, when adding silver nitrate, the amount of silver chloride that would leave a hexacoordinated cobalt was always obtained.
The coordination bonds of this type of compound have the characteristic of being colored.
In fact, they are responsible for the typical coloration associated with a metal (red iron, blue cobalt, etc.) and are important for atomic emission and absorption spectrophotometric tests (Skodje, SF).
References
- Atkins, PW (2017, January 23). Chemical bonding. Recovered from britannica.com.
- Clark, J. (2012, September). CO-ORDINATE (DATIVE COVALENT) BONDING. Recovered from chemguide.co.uk.
- Coordinate Covalent Bond. (SF). Recovered from chemistry.tutorvista.
- Coordination Compounds. (2017, April 20). Recovered from chem.libretexts.org.
- Introduction to Coordination Chemistry. (2017, April 20). Recovered from chem.libretexts.org.
- Jack Halpern, GB (2014, January 6). Coordination compound. Recovered from britannica.com.
- Schiller, M. (SF). Coordinate Covalent Bonding. Recovered from easychem.com.
- Skodje, K. (SF). Coordinate Covalent Bond: Definition & Examples. Recovered from study.com.