Polynucleating Species - Molecular Bigamists

When One is Not Enough

Small ligands like the didentate ethane-1,2-diamine may not satisfy the coordination sphere of a metal ion so that provided sufficient ligand is available the metal may choose to attach more than one ligand.  leading not only to ML species but ML2 and ML3 or even more attached ligands. One consequence of working with big ligands that carry many potential donor groups is that they may decide to do the exact opposite - use their suite of donor groups to attach to more than one metal ion at once. Thus, instead of a simple ML species, they may form M2L or even M3L or higher.

The whole situation is about getting satisfaction, or meeting the desires of both ligand and metal. In this marriage of metal and ligand, there needs to be some form of give and take to reach an agreeable outcome. The metal is seeking to maintain its desired number of metal-donor bonds, and the ligand is seeking to employ as many of its donor groups as practicable. They are after the same general outcome, but come to assembly with different demands and expectations. Normally, they reach a satisfactory match, which may be just a simple 1:1 assembly; however, it often really is a case of one is not enough. Satisfaction may require either the use of more ligands than metals or the exact opposite.

Let's look more closely at a situation where more than one metal ion could be accom- modate, with a relatively simple set of ligands. The polyaminoacid anion EDTA4 on the left in Figure 2.22 we have met already above, and it has a flexible aliphatic linker between the two N(CH₂COO-)2 heads. Thus (as described above), it can wrap up a single metal ion effectively, forming a 1:1 M:L complex species. However, the molecule on the right has the flexible linker replaced by a rigid, flat aromatic ring. Now the two nitrogen atoms cannot bend around to coordinate to two adjacent sites of the metal and thus cannot form the initial central chelate ring. Consequently this ligand has more success binding a single metal ion in each of its two N(CH₂COO-)2 head units; this leads to fewer donor groups coordinated to each metal, but this is the sole way that all six donor groups can be employed with a minimum number of metal ions, leading to a M2L complex species. Molecular bigamy has led to a successful compromise.

Figure 2.22

How ligand flexibility influences the process of complexation of polydentate ligands to metal ions. The rigid aromatic link in the right-hand example prohibits ‘wrapping’ of the two arms around a single metal, and coordination to two separate metal ions may be preferred.

Complexation of the ligand on the right in Figure 2.22 provides the ligand with a choice; it may form a 1:1 M:L complex and leave some donor groups uncoordinated, or form a 2:1 M:L complex and employ all donor groups. In reality both forms may exist in solution at once, depending on the relative concentrations of ligand and metal,

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مواضيع ذات صلة

?Preferences– Do You Like What I Like

Coupling– Polymetallic Complexes

A Separate Race - Organometallic Species

Supersized - Binding to Macromolecules

Vive la Difference– Mixed-metal Complexation

Many-Armed Monsters - Introducing Ligand Shape

Meeting More Metals - Bridging Ligands

More Teeth Stronger Bite - Polydentates

Greed is Good– Polydentate Ligands The Simple Chelate

Monodentate Ligands– The Simple Type Basic Binders

Ligands with More Bite– Denticity

Different Tribes– Donor Group Variation