Monodentate Ligands– The Simple Type Basic Binders

The simplest class of ligands offers a single lone pair for binding. The ammonia molecule (NH₃) is the classical example which we have in traduced above. Other members of the same column of the Periodic Table offer the same property such as phosphine PH3. However, their ability to act as ligands is related to their own inherent basicity stability and chemistry (PH3 is a weaker base than ammonia and is not stable in air or water, for example), and also to their meeting the right partner metal. Whereas N-donors are excellent choices for light first-row transition metal ions, P-donors are not as well suited, and bind better to heavier and/or lower-charged metal ions. These are concepts that we will return to later, but it does throw up one of chemistry’s conundrums– if water is 55 M in concentration and a useful ligand in its own right, why is it that even low concentrations of some other molecules dissolved in water can compete very effectively as ligands for a dissolved metal ion? We’ll return to this issue in Chapter 3.5. Havingmorethanonelonepairdoesnotexcludeamoleculefromactingasamonodentate ligand. We have already shown how water, with two lone pairs, can bind as a monodentate ligand to a metal ion. Once one lone pair is bound, the other points away in space and can only be employed, if at all, by a different metal ion. It is most usual to meet water as a simple monodentate ligand. In the extreme, a halogen ion has four lone pairs, but as all point to different corners of a tetrahedron, only one pair at a time can effectively interact with a particular metal ion, the others pointing away from it. Again, they are available for binding (through bridging) to other metal ions, but this is not a necessary outcome. All of R3N R2O and X− (X = halogen) ligands, with one, two and four lone pairs respectively, are met as monodentate ligands. The difference is that the latter two can also, in principle, bridge to other metal ions using their extra lone pairs, an option that ammonia does not have. However, even ammonia can achieve more lone pairs by the device of removing a proton, to form the NH₂− ion, which is then capable of bridging to two metal ions. Acting as an acid by releasing a proton is not our usual view of ammonia, because it is a base and losing a proton is not easily achieved; ammonia’s more usual behaviour is addressed below. Ammonia is basic and able to be protonated to form the ammonium (or azanium) ion, with its base strength defined by an experimentally measurable parameter the p_Ka. If base strength is considered as a measure of affinity for a proton, it seems reasonable to assume that this is also a reasonable measure of affinity for small, positively charged metal ions. This works reasonably well as a gross measure. For example, NH3 is much more basic than PH³, which can thus account for ammonia being a better ligand for first row transition metal ions than phosphine. Further, the oxygen analogue OH3+ (oxonium or oxidanium)

Figure 2.14

Examples of the N-donor family of monodentate ligands.

has one lone pair remaining but it is an extremely poor base and is not able to be protonated again to form OH4²⁺ (oxidanediium); it is also totally ineffective as a ligand to metal ions. We are now beginning to define some ligands as ‘better’ than others on the basis of certain chemical and physical properties– this is a move towards understanding selectivity and preference, or why molecules dissolved in a solvent can compete with that solvent as ligands for a metal ion.

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مواضيع ذات صلة

Meeting More Metals - Bridging Ligands

More Teeth Stronger Bite - Polydentates

Greed is Good– Polydentate Ligands The Simple Chelate

Ligands with More Bite– Denticity

Different Tribes– Donor Group Variation

Do I Look Big on That?– Chelate Ring Size

Putting the Bite on Metals– Chelation

Making Attachments– Coordination

The Road Ahead

Natural or Made-to-Measure Complexes

Metals in Contrived Environments

Metals in the Natural World