Ligand Field Theory - Making Compromises

A problem we left a little while ago in CFT was how to explain the experimentally observed order of ligands in the spectrochemical series and issues such as water being a better ligand than hydroxide and the neutral carbon monoxide being such a strong ligand. Molecular orbital theory has given us another perspective on the d-orbital set with the 12g level seen as dominantly nonbonding in character (which means that its energy should not be influenced particularly by changing ligands) and the eg level thus carrying the responsibility for altering the size of Δ_o as it responds to the presence of different ligands. Obviously, with just two energy levels involved, increasing the size of Δ_o can be done in two ways- either by raising the energy of the e* level or else by lowering the energy of the 12g level. Any action in the latter direction seems at odds with the 12g level being nonbonding in character or having little to do with the ligands. The crystal field model focused on raising or lowering of the eg level through electrostatic interactions between ligands and metal d electrons. Fortunately, the molecular orbital theory can provide us with a means for understanding how the 12g level may also be influenced and its energy manipulated by the ligands. If we return to the spatial orientation of metal orbitals in the 12g set we are reminded that they point not along the axes associated with ligand positioning, as for the 3d eg* orbitals or even 4s and 4p orbitals, but between the principal axis directions. Therefore, they can have little to do with traditional or-bonding (as by definition, a σr-bond is defined as one where electron density is enhanced in a direct line joining the two atom centres, i.e. metal and donor atom in the present case). The key to any interaction involving 2g d orbitals is whether the ligand donor has p or even π orbitals directed orthogonal (sideways-on) to the metal-donor bond direction. If this does occur, a further interaction between metal 2g d orbitals and ligand donor p (or π) orbitals can arise. In the usual MO manner, interaction of a single d orbital and a single p orbital would lead to two MOs, one bonding (π.or in-phase) and one antibonding (π *.or out-of-phase) orbital of lower and higher energy than the parent orbitals respectively - a mechanism for manipulating the energy of the 12g set has thus been established, and one that depends on the properties of the ligand. This is developed and exemplified in Section 3.3.4.

This capacity to model and account for the interaction of ligands capable of additional π-bonding with metals provides an enabling mechanism for dealing with the experimental spectrochemical series positioning of ligands. Of course, not all ligands will have the capacity to undergo further π-type interactions with metal orbitals. For example, ammonia has but one lone pair directed to o-bonding, and otherwise three internal N-H σ-bonds; it cannot undertake any further bonding. However, carbon monoxide (C=O) has an array of π-bonds resulting from its multiple bond character, and is a clear candidate for further π-type interactions (it is an example of a π-acceptor or π-acid ligand, a group of key importance in organometallic chemistry, with empty orbitals of symmetry appropriate for overlap with a filled d, or 12g, metal orbital). The scene has now changed, so that we have greater capacity to understand and predict the effects of ligands on the chemistry of metal complexes - this is the strength of the LFT.

What we have in the end is a reasonably consistent set of models, but ones that differ in their focus and assignment of importanc to electrostatic and covalent character. What is the 'real' situation, and how can we effectively assess the contribution of each compo- nent? A key and indeed fairly simple approach to this came forward from Pauling, who asserted that metals and ligands would adopt as far as practicable nett charges close to zero, through metal-ligand bond polarization or π-type metal-to-ligand or ligand-to-metal  electron density donation. As a consequence of this concept of electroneutrality, a metal ion in a high oxidation state with high formal positive charge would seek to involve itself in ligand-to-metal charge donation (i.e. acquisition of negative charge), whereas one in a low oxidation state may do the reverse. Support for this concept, experimentally, is to notice that a high-charged metal ion such as Mn(VII) finds itself dominantly with O₂ ligands, whereas lower-charged Mn(II) is satisfied with OH₂ ligands. What we have seen so far is that bonding between metals placed centrally in an array of atomic or molecular entities that participate in coordinate bonding can be represented by models of eventually high sophistication. Most of our understanding and interpretation of physical observables draws on these models, and it their success in allowing us to interpret what we see that keeps the models in use. But a model is imperfect because it is a model - and so higher levels of sophistication will reinvent or replace these models as time goes by. At present, at least, and for an introduction to the subject, they more than suffice - indeed they have proven remarkably resilient and effective, despite the extraordinary growth in coordination chemistry over the decades since their development. We shall examine bonding in terms of dealing with differen  geometries in the next section.

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مواضيع ذات صلة

Spontaneous Alternating Zwitterion Copolymerizations

Copolymerization of Cyclic Monomers

Ring-Opening Polymerizations of Cyclic Sulfides

Cationic Polymerization of Lactams

Polymerizations of Lactams

Special Catalysts for Polymerizations of Lactones

Special Catalysts for Polymerizations of Lactones

Polymerization of Lactones by Coordination Mechanism

Polymer-Based Solar Cells

Photo-Conducting Polymers That Are Not Based on Carbazole

Photoconductive Polymers Based on Carbazole

Photo-Conducting Polymers