Isolobal transition metal fragment

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. 

Not only can the P unit be coordinated terminally or bridging onto transition metals, but one or more of the phosphorus atoms in the P unit can be subshtuted by an isolobal transition metal fragment without destroying the cluster (see Fig. 7.44). Examples for monosubstitution of phosphorus by a 15 VE metal fragment can be... 

Aromaticity in compounds containing a transition-metal atom has already had a long history. Initially, Thom and Hoffmann [34] proposed six-membered ring met-allocyclic compounds, derivatives of benzene with one C-H moiety replaced by an isolobal transition-metal fragment. Three years later the first example of a stable. 

The addition of transition metal fragments ML (L = two-electron donor ligand) across formally unsaturated metal-metal or metal-carbon bonds is a well-developed synthetic route to heteronuclear clusters (1,2,11,12,27) and has received theoretical justification from Hoffmann s isolobal principle (46). The addition of a PtL2 fragment across an M=M double bond may be considered as analogous to the reaction of a carbene with an olefin, resulting in a cyclopropane. The use of isolobal analogies in the directed synthesis of heteronuclear clusters has been reviewed (11,12,27). 

Isolobal relationships between main-group and transition-metal fragments. Connections to organometallic chemistry... 

Isolobal main-group and transition-metal fragments... 

In the first three chapters, instances were noted where the number, symmetry characteristics and occupation numbers of the frontier orbitals of a transition-metal fragment were similar to those of a main-group fragment. Such fragments are said to be isolobal to emphasize similar bonding capabilities. Since its enunciation by Hoffmann and Mingos, the concept has been used effectively for the analysis of both organometallic and cluster problems. Let s explore the idea in a more systematic... 

Exercise 4.1. Fragment ethane into two CH3 radicals. Construct a neutral M(CO)m fragment isolobal with CH3. Give the structures and compositions of all compounds analogous to ethane that can be formed with these two isolobal main-group and transition-metal fragments. 

Some isolobal transition metal and main group fragments... 

Some isolobal transition metal ond main group fragments ch4 CH3 CHj CH C... 

It is often found that a transition metal fragment containing ten electrons more than a corresponding main group fragment has similar properties. This has been attributed to the 18 VE rule. However, a simple electron count is not enough to establish an isolobal relationship. For an isolobal relationship to be established, the molecular orbitals used to bind to other fragments (known as frontier orbitals) need to display a close similarity. 

The relationship between two organic fragments, CH and CH", and transition metal fragments are considered as the final example. The methylidyne fragment, CH, is isolobal with fragments such as Co(CO)3 and CpNi, and the CH" fragment is isolobal with fragments such as Fe(CO)j and CpCo. 

Many cluster molecules are formed by transition metal atoms which are coordinated to 7r-acid ligands, in particular carbon monoxide, cyclopentadienyl and phosphines. Under these circumstances the 18-electron rule should be obeyed so that a transition metal fragment has ten more valence electrons than the corresponding isolobal Main Group fragment. This relationship is exemplified in Table 11.3. 

Coordination chemistry of ER The monomeric fragments E-R are isolobal to carbon monoxide, and many complexes analogous to transition metal carbonyls have been synthesized (41 to 43, see Figure 2.3-7) [68], In most cases these reactions started with those clusters which have a high tendency to dissociate and to form monomers, such as pentamethylcyclopentadienylaluminum(I) or the alkylgal-lium(I) or alkylindium(I) derivatives. Often the products are isostructural to the respective metal carbonyls, but exceptions are the gallium compounds 44 and 45. 

Fig. 8. Isolobal relationship between methyl and methylene fragments, transition metal groups, and main group porphyrins.

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