Cumulene structure

Another, simple form of elemental carbon would be chains formed from carbon atoms. As a prototype model a single>stranded chain is most suitable. If branching were to be considered, all intermediate forms up to and including the diamond and graphite like clusters would be included. For non branched chains, the two variants to choose from are a system of alternating singly and triply bonded carbon atoms (poly-ynes), and a system with all double bonds (cumulenes). Cumulene structures are assumed to be the preferred ones for odd membered chains, whereas the even ones may have some poly-yne character. Recent studies on linear Cg show that a cumulene-like structure is preferred, both at the SCF level and when correlation is accounted for(50). 

The ferrocenyldiphynylpropargyl cation, 77, has an intrinsic delocalization nature exhibiting a valence tautomerization band at 856 nm, and its nucleophilic trapping reactions give rise to the formation of ferrocenyldiphyenylallenes (173). The bis(acetylide) mixed-valence complexes of ferrocene and the Ru complex moiety, 78, also behave as a fulvene-cumulene structure, 79, showing a u(M=C = C—C) band at 1985 cm-1 (174). Related alleylidene and cumulenylidene complexes of transition metals have been reviewed by Bruce (175). 

In order to complete the list of carbon oxides C =0 (n = 1-9) it should be mentioned that, as it was already discussed for C5O, also the higher members C7O, C80, and C9O have not been yet isolated in a matrix, but they could be generated by Endo et al.134,135 in the gas phase by an electric discharge in a mixture of C3O2 and argon. The reaction products were analyzed by rotational spectra, from which the linear cumulenic structure and the bond lengths were derived. 

I.3.4.2. Intermolecular Cycloaddition at C=X or X=Y Bonds Cycloaddition reactions of nitrile oxides to double bonds containing heteroatoms are well documented. In particular, there are several reviews concerning problems both of general (289) and individual aspects. They cover reactions of nitrile oxides with cumulene structures (290), stereo- and regiocontrol of 1,3-dipolar cycloadditions of imines and nitrile oxides by metal ions (291), cycloaddition reactions of o-benzoquinones (292, 293) and aromatic seleno aldehydes as dipolarophiles in reactions with nitrile oxides (294). 

Phosphaalkenes display a number of unusual properties. An investigation of phosphaalkenes with several aryl substituents, revealed that bis(diphosphene) 19 forms a triplet dianion211. A cumulene structure was demonstrated for the Ar-P = C=C=P-Ar radical anion. The unpaired electron is predominately localized on the P atoms212. 

The ultimate product obtained by oxidising cuprous acetylide could be formulated thus. Although infrared suggested a cumulene structure, not that shown above, it retains the hazards of its progenitor. 

Detailed DFT studies have been carried out on several C4 complexes,246,249,262 with the intention of understanding the electronic structures of these complexes and their redox-related analogues. The complex MnI(dmpe)2 2(p-C=CC=C) (93), was found to have a triplet ground state246 as predicted from the studies on the isoelectronic model compounds Mn(CO)2Cp 2(p-C=CC=C)396 and MnI(PH3)4 2(n-C=CC= C).246 Sequential oxidation of this compound serves to depopulate the 4n levels to give [93]+ and [93]2+ which possess more cumulenic structures, as is evidenced by the decreased v(CC) frequencies and the trends in C-C bond lengths observed in their solid-state structures. 

Redox reactions can induce significant redistribution of n electron density along the metal-carbon backbone when conjugated or cumulated carbon-rich ligands are present and may thus lead to major bonding changes. Particularly common are interconversions between alkynyl and cumulenic structures. 

Just as vinylidene-type resonance forms constitute one extreme description for the oxidised forms of alkynyl complexes (Scheme 6.4), cumulenic structures may result from the oxidation of oligoynediyl-bridged dimetal complexes (Schemes 6.5 to 6.8). Interesting examples come from the work of Sato, who... 

The first principles calculations performed by Kiirti et al. have shown Raman bands of both polyyne and cumulene carbon chains to occur at around 2000 cm [22]. The authors emphasized that distinguishing on the basis of Raman spectra between the polyyne and cumulene structures appears to be unlikely. 

Kiirti, J. Magyar, C. Balazs, A. Rajczy, P. Vibrational analysis for short carbon chains with alternating and cumulenic structure. Synth. Met. 1995, 77(1-3), 1865-1866. 

Linear C species may be represented simply as shown in Fig. 1-6. For an even number of carbons, the simplest electronic structure may be either a dicarbene-cumulene structure, or a diradical-polyyne . The corresponding cyclic structures will be nonlinear and strained, but formally possess closed-shell cumulene or polyyne structures. These differ by having all bond lengths equal, or alternating bond lengths, respectively. For it = odd, the linear structures may be of the dicarbene-cumulene or tetraradical-polyyne type. The cyclic isomers may be cumulene or carbene-polyyne . 

CISD calculations (DZP basis set) for several cyclic and linear isomers of Cio (see Fig. 1-10, 8-12) were performed by Liang and Schaefer [262]. They concluded that (i) the monocyclic forms of Cio are considerably more stable than the linear structures (ii) the 0, symmetrical cumulenic structure 9 is the ground state, but electron correlation decreases the energy difference between 9 and the alternative ring structures 8 and 10 and (iii) the linear acetylenic structure 12 is just slightly less stable than the linear cumulene 11. 

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