Isolobal analogies

Key point: Structurally analogous fragments of molecules are described as isolobal; groups of isolobal molecular fragments are used to suggest patterns of bonding between seemingly unrelated fragments, allowing the rationalization of diverse structures.

We can identify analogies in the structures of apparently unrelated molecules. Thus, we may view N(CH₃)₃ as derived from NH3 by substitution of a CH₃ fragment for each H atom. In current terminology, the structurally analogous fragments are said to be isolobal, and the relationship is expressed by the symbol. The origin of the name is the lobe-like shape of a hybrid orbital in a molecular fragment. Two fragments are isolobal if their highest energy orbitals have the same symmetry (such as the σ symmetry of the H1s and a Csp3 hybrid orbital), similar energies, and the same electron occupation (one in each case in H1s and Csp3). Table 22.10 lists some selected isolobal fragments and the first line shows iso lobal fragments with a single frontier orbital. The recognition of this family permits us to anticipate by analogy with H−H that molecules such as H₃ C−CH₃ and (OC)₅ Mn−CH₃ can be formed. The second line of Table 22.10 lists some isolobal fragments with two frontier orbitals, and the third line lists some with three. Isolobal analogies provide a good way to picture the incorporation of hetero atoms into a metal cluster. These analogies allow us to draw a parallel between [Co₃(CH)(CO)₉] (78)

and [Co4 (CO)₁₂] (76), both of which can be regarded as triangular Co₃(CO)₉ fragments capped on one side either by Co (CO)₃ or by CH. A minor complication in this comparison is the occurrence of Co(CO)₂ groups together with bridging CO ligands in [Co4 (CO)₁₂] because bridging and terminal ligands often have similar energies. Further inspection of the isolobal fragments given in Table 22.10 shows that a P atom is isolobal with CH; accordingly, a cluster similar to [Co₃(CH)(CO)₉] (78) is known, but with a capping P atom. Similarly, the ligands CR2 and Fe (CO)₄ are both capable of bonding to two metal atoms in a cluster; CH₃ and Mn (CO)₅ can bond to one metal atom. As a final example of isolobal analogies, consider the mixed manganese and platinum complex (79): the three-membered {Mn, Pt, P} rings can be considered as the metallocyclo propane form of a coordinated double bond (80). Both halves of the Mn P fragments are isolobal with CH₂ if we treat them as PR2 and Mn (CO)₄; the complete fragment can then be treated as analogous to an ethene molecule. This treatment means that (79) can be considered analogous to bis-(ethene) carbonyl platinum (0) (81), a known simple 16-electron organometallic compound.

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مواضيع ذات صلة

Structure of clusters

Bent metallocene complexes

Fluxional behaviour of metallocenes

Bonding in bis(cyclopentadienyl)metal complexes

Synthesis and reactivity of cyclopentadienyl compounds

Spectroscopic properties of carbonyl compounds

Reactions of the CO ligand

Metal carbonyl basicity

Oxidation and reduction of carbonyls

Properties of homoleptic carbonyls

Synthesis of homoleptic carbonyls

Homoleptic carbonyls