Covalent Bonds: Coordinate Covalent

Up to now, we have seen three different types of bonds: ionic, polar covalent, and nonpolar covalent. However, there is one last special type of covalent bond left to explore, the coordinate covalent bond.

Who Is Lewis Anyway?

Coordinate covalent bonds are special in that the bonding electrons originate from one of the two atoms involved. In previous covalent bonds, the bonded atoms each atom brought one electron to the table. Not so here, since one atom will end up providing both electrons required to form a bond.

These bond form between what are called Lewis acids and Lewis bases. Definitionally this makes sense since a Lewis acid is an electron pair acceptor and a Lewis base is an electron-pair donor. Basically, the Lewis base gives its electron pair to a Lewis acid forming a coordinate covalent bond in the process.

Coordination Complexes

Coordinate covalent bonds result in coordination complexes, where a central atom ends up bound by multiple atoms or molecules called ligands. Typically the central atom is a transition metal, I like to remember this by recalling that transition metals tend to carry decently large positive charges (Fe³⁺ of Cr⁶⁺). This means that they don’t have any electrons to give so they need their bonding partner to provide them all.

Similarly, we can use this idea to recognize what molecule or atoms act as ligands. Since ligands need to bring all the electrons to the bonding party they have to have them in the first place. Atoms and molecules with lone pairs, unbonded electrons that surround an atom, make excellent ligands. Examples include halogens, nitrogen, oxygen, and sulfur.

Heme: An Example

Coordination complexes abound in biochemical processes. We often see these organometallic coordination complexes acting as cofactors for enzymes, as ionophores in ionic channels, and notably as the heme center in hemoglobin.

Let’s take a look at heme quickly to put the ideas we have seen above into context. Here the transition metal iron acts as the central atom. Bonded to it are a multitude of nitrogens who have donated their lone pairs. Oxygen gas with its lone pairs can bind iron and as result be carried off to the cells within our body.

How Many Teeth?

In heme, we see a specific type of ligand called a chelator. A chelator because the same molecule is bound to the central atom multiple times. We also call chelators, polydentate ligands. Poly meaning many and dentate meaning teeth so a polydentate ligand is a ligand with multiple teeth or bonds to the same thing.

In order to keep track of all of the ligands and bonds, we can define the number of ligands and the coordination number of a complex respectively. So if we count the number of ligands on heme we can see there are three as indicated by the three different monsters latched onto iron above. However, the central iron atom has six bonds and therefore a coordination number of six.

These numbers don’t match up because even though we have three different types of molecules they are able to form a total of six bonds. So in summary the ligands refer to the individual molecules that bond with the central atom while the coordination number defines the number of bonds that the central atoms has.

For the MCAT there are only two bidentate molecules( which form two bonds to the central atom) we need to memorize and recognize. The first is ethylenediamine, most commonly abbreviated en, and the second is oxalate, abbreviated ox. Every other ligand you encounter in a molecular formula is monodentate unless the passage indicates otherwise. Make sure to check any figures to determine a molecule’s dentition by counting how many bonds that molecule forms.