A Separate Race - Organometallic Species

We have mentioned earlier the prospect of the carbanion H₃C being an effective ligand, since it offers an electron pair donor in the same way that ammonia does. To emphasize this aspect even simple compounds like [M(CH₃)(NH₃)₅]"⁺ have been prepared in recent decades. A vast area of chemistry has grown featuring metal-carbon bonds called, not surprisingly, organometallic chemistry. Typical ligands met in this area are carbon monox- ide, alkenes and arenes; it is also conventional practice to include hydride and phosphine ligands. There has been a tendency to regard this field as somehow separate from traditional compounds, but the division is somewhat artificial, although we will see there are some good reasons for this schism. One simple distinction is that, unlike the usually ionic Werner- style compounds, many organometallic compounds are neutral, low melting and boiling point compounds that dissolve in organic solvents, and display greater ligand reactivity. Further, they usually feature metals in low oxidation states, and in polymetallic systems are more likely to involve direct metal-metal bonds. They can also display ligand types and structural characteristics that were not anticipated from classical Werner-style coordination chemistry, and that require more sophisticated models for satisfactory interpretation.

One distinction that we meet relates to the mode of bonding in organometallic com- pounds. We have seen how H3C can make use of its lone pair in the conventional manner to coordinate, forming a σ covalent bond. This approach is less obvious for a molecule like ethene (H₂C-CH₂), which has been shown to usually involve side-on bonding with the two carbon centres equidistant from the metal centre. This is not an arrangement easily dealt with by the simple covalent bonding model which can accommodate bonding only by regarding the molecule as formally H₂C^--⁺CH₂ with the carbanion carrying the lone pair alone able to bond covalently. There are clearly problems with this approach, since the bonding arrangement in the resulting complex involves a single or bond leaving the

Figure 2.25

Coordination of H3C and H2C=CH2 to metal centres. Whereas the former can be interpreted using a traditional o covalent bonding model, this is far less successful for the latter.

carbocation displaced away from the metal centre; even allowing an equilibrium to operate between the two carbon sites the model is not very compelling. What is found in reality is that not only is each carbon equidistant from the metal centre but also the carbon-carbon bond length when coordinated remains much like a standard double bond, with the whole ethylene molecule remaining flat and showing no tetrahedral distortion (Figure 2.25). It appears that some form of π bonding may be involved, and a different bonding model involving a molecular orbital approach is required.

The 'side-on' bonding situation is exaggerated further when coordinating one of the classical ligands of organometallic chemistry the cyclopentadienyl ion, CsH₅ (Cp ̄) (Figure 2.26). This organic anion is formed by deprotonation of cyclopentadiene; in a 'frozen' format, it could be considered to have one carbon with a lone pair, which could bind to a metal ion through this single carbon in a simple covalent or-bonded mode. However, this is not the only or even usual form of coordination. Rather, it commonly binds to a metal ion 'side on', with all five carbon atoms equidistant from the metal ion, in what can be considered a π-bonding mode. In this mode of coordination it effectively occupies a face of an octahedron, in a somewhat similar manner to how a cyclic, saturated triamine ligand can do. In Figure 2.26 (where H atoms are usually not shown for simplicity), comparison is made between a metal ion bound to two cyclic triamine ligands, one each of a triamine and a cyclopentadienyl anion, and two cyclopentadienyl anions. You may see how the Cp anion effectively replaces three traditional o-donor groups on a 'face', leading eventually to a [M+(Cp)2] compound called, for obvious reasons, a 'sandwich' complex. The Cp and other unsaturated carbon-bonding ligands can exhibit variable coordination through the use of different numbers of available carbons (in the same way that a traditional polydentate ligand may not employ all of its potential donor groups in coordination) and the mode of coordination is defined by an nomenclature, where the superscript x represents the number of carbons in close bonding contact with the metal atom. For example the mode with a single M-C bond shown below would be a n' bonding form, whereas the usual facially-bound form would be an bonding mode.

Apart from unsaturated alkenes and cycloalkenes acting as ligands the simplest ligand of organometallic chemistry is carbon monoxide, which usually acts as a monodentate σ- bonded ligand through the carbon atom, forming a linear M-C=O bonding arrangement, although our covalent bonding model sits uneasily with this representation and a molecular

Figure 2.26

Formation and coordination of H5C5− as a-bonded ligand and as a-bonded ligand. In the latter mode, the ligand occupies the face of an octahedron in the same way that a conventional cyclic triamine does.

orbital model is more useful. In this field we also see an example of a rare cationic ligand NO+. With organometallic chemistry such an extensive and demanding field in its own   right, we shall deliberately limit our discussion of these somewhat unusual ligands and their compounds in this introductory textbook. However, there are aspects of their mode of  coordination and spectroscopic properties that have driven extension of bonding models .

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مواضيع ذات صلة

?Preferences– Do You Like What I Like

Coupling– Polymetallic Complexes

Supersized - Binding to Macromolecules

Vive la Difference– Mixed-metal Complexation

Polynucleating Species - Molecular Bigamists When One is Not Enough

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