magnificentmolecules

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A macrocyclic magnesium complex

A macrocycle is a cyclic molecule usually containing at least three donor atoms (atoms with lone pairs of electrons in their valence shells), and often able to fully encircle a metal ion.

The picture shows cyclam (a portmanteau of ‘cyclic’ and ‘amine’, its two major chemical traits) coordinating to a magnesium ion with the lone pairs of each of the four nitrogen atoms. All hydrogens are omitted from the model for simplicity, but here is the full structure:

Compounds with polydentate ligands (multiple donor atoms per ligand), such as the above cyclam, are much more stable (both thermodynamically and kinetically) than their monodentate analogues. For example, four ammonia molecules coordinating to a magnesium ion would be less stable, as each molecule is monodentate (only has one donor atom). This is called the chelate effect.

A similar effect (the macrocyclic effect) describes the additional stability of a cyclic ligand over an acyclic one.

In the above model, the magnesium has adopted a tetrahedral shape to maximise orbital overlap, but it should be noted that the Mg-N bonds aren’t covalent bonds, they’re ionic interactions that show how the high electron-density on the nitrogen is attracted to the positive charge on the metal ion, forming an energetically favourable bonding interaction that holds the ion in place.

One last thing: the magnesium ion has a +2 charge, and the cyclam isn’t charged. Although the coordination of the ligand has stabilised the charge by delocalising it, the whole complex still has a charge of +2 that must be balanced by anionic counter-ion(s) with a net charge of -2, for example oxygen, or two chlorines.