Vinylidene carbenes

Key words DFT, isomerization, alkene, alkyne, carbene, vinylidene. 

The metal-ligand fragment L M, the number of carbon atoms x, and the substituents at the terminal sp -carbon may vary considerably and, correspondingly, the properties and reactivities. The early members of the series of cumulenylidene complexes (x=l, 2, 3 carbene, vinylidene and allenylidene complexes) have established themselves as invaluable building blocks in stoichiometric synthesis and as highly potent catalyst precursors. The higher members might potentially be very useful candidates for application as one-dimensional wires and in opto-electronic devices. 

Values of those parameters have already been proposed for many ligands (Ei) and a considerable number of metal centers (S m and 7m), mainly by Lever [64-67, 72] and occasionally by others [14, 15, 24, 27, 29, 35, 38, 41, 48, 71, 73-77]. Examples concerning the former parameter are listed in Table 9 and an extensive list for many carbyne, carbene, vinylidene, allenylidene and alkynyl ligands has been proposed recently by Pombeiro [38]. Tables 10-12 list the proposed S m and 7m values for the binding metal centers so far studied. 

Complexes containing the unsaturated carbenes vinylidene (G and H) and propadienylidene (allenylidene) (I and J), and their cluster-bound analogs, are also known, although general routes to these reactive compounds have been discovered only within the last five years or so. Vinylidene, the simplest unsaturated carbene, has never been observed experimentally, as it undergoes an extremely fast 1,2-hydrogen shift to give acetylene ... 

Since, in transition metal complexes, the carbene (vinylidene) form of acetylene is often stabilized, it is quite possible that in the presence of alkali metal cation this form also becomes more stable and makes a certain contribution to activation of the triple bond. 

This model has proven very effective for a wide range of Werner-type classical and organomet-allic species. Some problems arise in the latter class where stereochemistry can be a factor, i.e., isomers of different stereochemistry having rather different redox potentials. This appears to be particularly troublesome with certain strongly 7r-bonding ligands such as carbonyls, isonitriles, carbenes, vinylidenes, In these cases it is preferable to adopt a correction following... 

One more reaction, for which the crucial role of the tunnelling mechanism was demonstrated, is the 1,2-sigmatropic hydrogen shift in unsaturated carbene vinylidene XLIX—a prototype of unsaturated carbenes ... 

Three papers report thermally-generated carbenes. Vinylidenes have been generated from cyclopropenes, such as (82), by thermolysis. The vinylidene (83) rearranged to the corresponding alkyne (84). The proposed vinylidene intermediate was shown, by calculation, to be an energy minimum. The energy barriers are within reach of the substrate under the conditions needed to generate the vinylidenes and the barriers are lower than those for the alternative radical pathways. ... 

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