The Isolobal Analogy

In many cases, we established the orbitals of ML fragments earlier in this book by starting If om a complex in which one or several ligands were removed. For example, the orbitals of ML5 complexes (SBP, 4 ), ML4 ( butterfly , Czv), and ML3 (pyramidal, C y) were obtained from those of an octahedral ML5 complex, by removing one, two, or three ligands, respectively (Chapter 2, 2.3.1, 2.8.1, and 2.8.3). In certain respects, each of these complexes was therefore considered as a fragment of an octahedron (5-1). 

The same procedure can be applied to molecules of the type AH , where A is an element from the second or third row of the periodic table. In particular, starting from tetrahedral methane, CH4, the archetype of organic molecules, one can obtain the organic fragments pyramidal CH3(C3v), bent CH2(C2v), and CH(Coov), by removing one, two, or three atoms ofhydrogen (5-2). 

Even though these comments are purely geometrical in nature, it turns out that they have profoimd chemical consequences. Thus, an ML5 complex (SBP) possesses a. vacant site, and it is likely to bind an additional ligand to form an octahedral complex. In the same way, the ML4(C2v) and ML3(C3v) fragments have two and three available sites, respectively, with respea to the initial octahedron. In the organic series. 

Pauling The Nature of Chemical Bond, 3rd edn., Cornell University Press, Ithaca, NY 

Using the isolobal analogy, we can understand a number of different organometallic complexes. Any species with a single electron should be isolobal with methyl. This not only includes cf ML5 species, but also (f ML4 species (17-electron complexes). As an example of a ML4 species, Co(CO)4 is isolobal with methyl. It combines with alkyl groups and itself. Furthermore, CpFe(CO)2 is isolobal (17-electron species). Indeed, CpFe(CO)2 dimerizes and makes single bonds to alkyl fragments. 

The utility of the isolobal analogy is that if a ML fragment is isolobal with a particular. AH arrangement, then (me should be able to replace the ML, fragment in a molecule with a AH unit to produce a new compound (on paper at least). The two molecules should have a very similar electronic description and in a very qualitative sense have similar reactivity. The isolobal analogy is, therefore, a useful 

Inorganic fragment M o Otgaaic Fragment Complex Isolobal with 

FIGURE 13,5 Stepwise isolobal replacement of CH by CofCO) in tetrahedrane. Co4(CO)i2 has the CO bridged structure shown. 

Stnumire 13 is usuaUy considered as a bridging aikyne complex of Co2(CO)s, and 13.22 as a cyclopropenyl complex of Co(CO)3. The all-carbon compound 13.21, is unstable and reverts to two molecules of acetylene, but stable tetrabe-dranes C4R4 have been made by using very bulky R groups. 

Any bridging hydrides can be removed as protons for example, the dinuclear hydride in Eq. 13.11 is isolobal with acetylene because the ISe IrHLs fragment has three holes and three orbitals. CO ligands contribute in the same way whether they are bridging or terminal (e.g., Eq. 13.12), but the rhodium dimer (Eq. 13.13) has bridging CO groups. 

Isolobal ideas give us a way to draw on analogies b ween (uganic and inorganic structures. 

Hoffmann s isolobal analogy is a general unifying principle that goes far beyond the confines of cluster chemistry. Nevertheless it has found most 

Many metal cluster complexes were originally synthesized by unplanned routes, or as by-products in other reactions. Only recently have systematic procedures been developed for making metal-metal bonds and building up clusters. 

By pyrolysis of mononuclear carbonyl complexes (it appears that CO is lost first, and the unsaturated fragment then attacks the original carbonyl)  

An important contribution to the understanding of parallels between organic and inorganic chemistry has been the concept of isolobal molecular fragments, described most elaborately by Roald Hoffmann in his 1982 Nobel lecture. Hoffmann defined molecular fragments to be isolobal 

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. 

These fragments can be combined into molecules. For example, two CH3 fragments form ethane, and two Mn(CO)5 fragments form (OC)5Mn—Mn(CO)5. Furthermore, these organic and organometallic fragments can be combined into H3C— Mn(C0)5, which is also a known compound. 

The organic and organometallic parallels are not always this complete. For example, although two 6-electron CH2 fragments form ethylene, H2C==CH2, the dimer of the isolobal Fe(CO)4 is not nearly as stable it is known as a transient species obtained photochemically from Fe2(CO)9. However, both CH2 and Fe(CO)4 form three-membered rings, cyclopropane and Fe3(CO)i2. Although cyclopropane is a trimer of 

FIGURE 15-3 Structures Resulting from Combinations of Isolobal Co(CO)j and CR. 

The parent compounds have filled valence shell electron configurations, an octet for CH4, and 18 electrons for MLs (Cr(CO)g is an example of such an MLg compound). Within the hybridization model, methane uses sp hybrid orbitals in bonding, with 8 electrons occupying bonding pairs formed from interactions between the hybrids and Is orbitals on hydrogen. The metal in ML5 uses (fsp hybrids in bonding to the ligands (the and 

Os3(CO)i2, on the other hand, is a trimeric combination of three Os(CO)4 fragments, which are isolobal with both Fe(CO)4 and CH2, and can correctly be described as [Os(CO)4l3. 

Hoffmann s isolobal analogy is a unifying principle that identifies analogies between organic and inorganic structures in terms of their bonding pattern. Different L M groups are considered isolobal with 

Cr(CO)5 with one empty orbital having two holes is isolobal with CH3+. Just as Cr(CO)5 reacts with CO to give Cr(CO)6, CH3+ reacts with CO to give the acetyl cation CH3CO, a Friedel-Crafts intermediate, formed from MeCOCl and AICI3. 

The CH2 fragment can use its two orbitals and two holes quite flexibly by changing its hybridization. If CH2 binds two H atoms to give CH4, sp hybridization now applies, but if it binds to a second CH2 to give C2H4, then sp hybrids form the a bonds and a p orbital forms the t bond. Mo(CO)5 is isolobal with CH2 because it also has two orbitals and two holes. The empty orbital is the 2e vacancy at the metal and the other is a orbital normally involved in back bonding. Fischer car-benes (CO)5Mo=CR2 show how the Mo(CO)5 fragment replaces one CR2 in the alkene R2C=CR2. 

Inorganic Fragment o Organic Fragment Complex Isolobal with 

The isolobal relationship is symbolized by a double-headed arrow with a teardrop, as shown in 21.1. Note that the first three criteria are absolute while the last two are qualitative comparisons. This isolobal analogy is a model designed to 

Thomas A. Albright, Jeremy K. Burdett, and Myung-Hwan Whangbo. 

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Taking into account the isolobal analogy between RSb and 16-e ML fragments metallocycles, the complexes of type 25 may be considered as analogs of cyclo-tristibine systems. Type 26 is related to complexes with cyclo-tristibine ligands. 

The isolobal analogy between the imido and Cp ligands makes the Cp/imido ancillary a potential platform to study the IHI (see 105 and Scheme 5). 

Schmidbaur and coworkers have prepared the pentaaurylated carbomonocation (4), an isolobal analog of CH and hexaaurylated carbodication (5), the isolobal analog of From X-ray crystallographic studies, it was shown that the... 

A theoretical approach addresses the question of alkynes bonded to PtL2 fragments in both parallel and perpendicular geometries. With each mode of alkyne coordination there is required a different coordination geometry at platinum. The authors use the isolobal analogy to calculate the electronic structures of complexes, and propose several unknown complexes to be stable.842... 

This property of gold(I) moieties to form partially bonding contacts to other gold species has been described as aurophilicity by Schmid-baur and his co-workers (266) and represents an important limitation of the isolobal analogy. An example of this is provided by a comparison of the structures of [H2Fe(CO)4] (261) and [Fe(CO)4(AuPPh3)2] (51), which are illustrated in Fig. 18. 

Using the Isolobal Analogy Metal Complexes with Bridging Carbenes and Carbynes... 

Scheme 8.15 Stone s synthetic route to obtain dinuclear and trinuclear transition metal complexes such as 58 (with M (L) — Pt(PR3)2, (C5Me5)Rh(CO), (CgMe6)Cr(CO)2, Fe(CO)4, etc.) and 59 using the isolobal analogy (a R = H, b R = Me R = p-Tol)...

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