Isolobal relationships

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

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

Isolobal relationships between metal and nonmelal fragments will undoubtedly continue to be exploited and intense activity can be expected in this area for some time to... 

This adduct is also formed by the salt-elimination method shown in Scheme 3 (144). The structure of XXXVI has been confirmed by a crystallographic study (143). The structure of XXXVIII is unknown so far, but following the isolobal relationship between the fragments L and M it is not unreasonable to suggest that a structure similar to that of the stannacarborane bipyridyl adduct (145) will be present, as indicated in Fig. 8. 

Using the isolobal relationship between CH or CH2 units and corresponding transition metal or main-group element fragments, the majority of the nonclassical carbocations can be theoretically treated as n complexes (30,186-188). This is demonstrated for some representative examples in Scheme 4. The description of these species, which have not been synthesized from positively charged CH or CH2 fragments and organic n systems, is beyond the scope of this review. 

Isolobal Relationship Two molecular fragments are isolobal if the number, symmetry properties, shapes and approximate energies of their frontier orbitals are the same. They may or may not also be isoelectronic. If the two fragments are isoelectronic, in the sense that the ratio of the number of electrons in frontier orbitals to the number of frontier orbitals is same, then the net charges on the two species will be the same. The orbitals whose similarity is critical in determining isolobality are called frontier orbitals. 

Complex 2 is a potent cluster precursor. The isolobal relationship (194) between diarylalkynes and the alkylidynes CpW(CO)2(=CR) is now quite well known (195). Since 2 and 3 readily add alkynes, one might reason that alkylidynes CpW(CO)2(=CR) will also add across the M=M bonds of 2 or 3. In fact, this happens, and in quantitative yield for 2, when the product is 73. The yield of 74 in the tungsten reaction is less impressive, but it is obtained in higher yield in the reaction of Cp2W2(CO)4-(fi-rf-RCCR) with CpW(CO)2(=CR). The behavior of the chromium system is quite different, the product being the /x-alkyne species Cp2-W2(CO)4(jU-i72-RCCR). Remarkably, 1 is catalytic for this dimerization (196). 

An isolobal relationship exists between 5 and ethene. The logic goes that it should therefore be possible to construct molecules containing the Cp2-Rh2(CO)2 unit that are formally analogous to ethene complexes. In practice, 5 is used because of the essential nonavailability of the Cp species. Many compounds have been prepared by using this approach (195,236). 

Fig. 2. Structures of the clusters [AuOs3(/i-H)(CO)10(PR3)] (R = E 1 °r Et) and [Os3-(/x-H)2(CO)10], showing the isolobal relationship between the edge-br dging Au(PR3) unit and the hydrido ligand.

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