Cumulenylidene complexes/ligands

C. Ruthenium Cumulenylidene Complexes Bearing Bis(pyrazol-1-yl)acetato Ligands... 

Cumulenylidene complexes constitute a class of organometallic compounds in which a chain of sp-hybridized carbon atoms is terminated by a sp -hybridized carbon atom (CR2 group) at one end and by a metal-ligand fragment (Lr,M) at the other end. 

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. 

Transition metal complexes containing allenylidene, cumulenylidene, and related ligands with heterocyclic fragments 98CRV2797. 

Keywords Allenylidene ligands Carbene complexes Cumulenylidene ligands Pentatetraenylidene ligands Propargylic alcohols... 

Structural parameters of selected mononuclear complexes containing heteroatomic 7i-donor-substituted allenylidene ligands are summarized in Table 1. When these data are compared to those reported for related complexes bearing conventional alkyl or aryl substituents (see Table 1 of chapter All-Carbon-Substituted Allenylidene and Related Cumulenylidene Ligands ), a lengthening of the M=C bond... 

In complexes vith an even number of carbon atoms in the chain the HOMO is oriented perpendicularly to the plane of the cumulenylidene ligand and the LUMO is an in-plane orbital. Conversely, in complexes with an odd number of carbon atoms the HOMO is an in-plane orbital and the LUMO is perpendicular to the plane [10, 12[. 

These observations indicate that when the metal complex is electron-rich, the allenylidene-metal complexes are stable (VI and VII), even on heating or protonation [42]. However, with less electron-rich systems (e.g., PPh3 ligands instead of PCy3 or NHC) the corresponding allenylidene complex was never observed, to the profit of the indenylidene complex VIII. These results suggested that the allenylidene-ruthenium complex is a transient species that rearranges into the indenylidene complex VIII, as was observed for a C5 cumulenylidene [48]. 

M. I. Bruce, Transition Metal Complexes Containing Allenylidene, Cumulenylidene, and Related Ligands, Chem. Rev. 98, 2797-2858 (1998). 

Ligands with chains of carbon atoms that have cumulated (consecutive) double bonds, designated cumulenylidene ligands, are also known. Such metallacumulene complexes have drawn interest because of possible applications as 1-dimensional molecular wires and for nse in nanoscale optical devices. In recent years, complexes with 2- and 3-carbon chains have also been developed as effective catalysts. Metallacumulene complexes with carbon chains containing three or more atoms have recently been reviewed. ... 

The longest cumulenylidene ligand reported to date is the heptahexaenylidene complex shown in Figure 13.49. As in the case of extended organic pi systems, the difference in energy between the HOMO and the LUMO decreases as the length of the cumulene ligand increases. ... 

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