Isolobal pairs

Using the isolobal principle, one can relate clusters that contain fragments having analogous orbital properties. Some isolobal pairs of metal carbonyl and hydrocarbon fragments are ... 

Figure 1-23 Some simple examples of isolobal species. Each pair is represented in two ways above, with least hybridization below, with fullest hybridization.

Slee, Lin, and Mingos have analyzed the geometries of small silicon clnsters, exploiting a coimection with the deltahedral boranes and carboranes. A silicon atom with an ont-pointing lone pair and three in-pointing hybrids is isolobal to a B-H vertex. However, Si clusters have 4n valence electrons, two fewer than the corresponding boranes, B H . Hence, we shonld not expect the two series of clusters to be isostructural, but it may be helpful to start... 

Similarly, 6-electron CH2 and 16-electron ML4 are isolobal. Both CH2 andML4 are 2 electrons short of a filled shell octet or 18-electron configuration, so they are electronically equivalent each has 2 single electrons occupying hybrid orbitals at otherwise vacant sites. Absence of a third ligand similarly gives a pair of isolobal fragments, CH and ML3. 

The absence of a third ligand from the parent polyhedra also gives a pair of isolobal fragments, CH and ML3. 

CH units of tetrahedrane have been replaced by isolobal " metal residues. However, in the pentagonal pyramidal skeleton of [Fe(CO)3]2(CMe)2(COH)2, ° if all of the skeletal carbon-carbon and metal-carbon links are assigned a pair of electrons, then the metal-metal bond in this compound has to be regarded as a dative bond from the apical iron atom to the basal iron atom. 

E22.25 (a) Co3(CO)9CH]— OCHj, N(CHj)2, or SICHj, Isolobal groups have the same number and shape of valence orbitals and the same number of electrons in those orbitals. The CH group has three sp hybrid orbitals that contain a single electron each. The SiCHj group has three similar orbitals, similarly occupied, so it is isolobal with CH and would probably replace CH in [Co3(CO)9CH] to form [Co3(CO)9SiCH3]. In contrast, the OCHj and N(CH3)2 groups are not isolobal with CH. Instead, they have, respectively, three sp orbitals that contain a pair of electrons each and two sp orbitals that contain a pair of electrons each. 

In Fig. 2.4 we can follow the trend throughout the phosphorus chemical shift range. At the upheld end, white phosphorus resonates at < p= 88ppm. This is readily explained by an electron rich cluster structure with P-P-P angles of 60°. Coordination to a transition metal causes a reduction in electron density on phosphorus, as a P lone pair is used to create a donor bond. Similarly, substitution of a phosphorus atom by an isolobal transition metal fragment causes a downfield shift, as electron density is transferred from the electron rich phosphorus atoms to the poorer transition metal. 

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