Propadienylidene

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 ... 

The first propadienylidene complexes were reported in 1976 (16,17), and examples of terminal and bridging ligands of this type are now known their chemistry is now beginning to be understood. This article is concerned with the synthesis, properties, and chemistry of transition metal complexes containing these unsaturated carbene ligands. 

The acylynolate anion (50) can be decarbonylated on irradiation, and subsequent deoxygenation (formal loss ofO2-) affords the propadienylidene complex (22). In some cases, this is not isolable or spontaneously polymerizes, but its intermediacy can be inferred from products obtained when the... 

The anionic complexes (50) are obtained from the group VI carbonyl and the dilithium derivatives of the hydroxyalkynes HC=CCR2(OH) subsequent protonation or acylation affords propadienylidene (51) or carbene (52) complexes by complex cyclization and addition reactions (Scheme 3) (28). Attempts to obtain the dimethyl complex by reaction of the chromium alkynolate dianion with COCl2 gave only polymeric material. More stable complexes were obtained with R = aryl (73). 

These reactions are analogous to those observed for the corresponding vinylidene complexes the second propadienylidene group is lost, presumably as R2C=(C=C)2=CR2 only in one instance has this hydrocarbon been detected in the thermal decomposition of a mononuclear complex (77). The tendency for the propadienylidene residue to bridge two metal atoms is so great that the binuclear complexes are often formed in reactions designed to generate the mononuclear derivatives (73). 

The majority of the chemistry of vinylidene and propadienylidene complexes is concerned with their synthesis and reactions, which have been... 

L are singlet states, the latter having two unpaired but spin-paired electrons form M is a triplet with the two unpaired electrons having parallel spins. In contrast with CH2, which has a triplet ground state, theoretical calculations and experimental results are both consistent with a singlet ground state for H2C=C . Propadienylidene, H2C=C=C , is considered to be a hybrid ... 

Tables I and II summarize the structural studies of mononuclear and binuclear vinylidene complexes, and Table III those of propadienylidene complexes which had been reported to mid-1982. As can be seen, the C=C bond lengths range from 1.29 to 1.38 A, and the M-C bond (1.7-2.0 A) is considerably shorter than those found in alkyl or simple carbene complexes. Both observations are consistent with the theoretical picture outlined above, and in particular, the short M-C bonds confirm the efficient transfer of electron density to the n orbitals. In mononuclear complexes, the M—C=C system ranges from strictly linear to appreciably bent, e.g., 167° in MoCl[C=C(CN)2][P(OMe3)2]2(fj-C5H5) these variations have been attributed to electronic rather than steric factors. In the molybdenum complex cited, the vinylidene ligand bends towards the cyclopentadienyl ring (111).

All known vinylidene and propadienylidene complexes are listed in Tables IV-VII, together with some salient physical properties. In their IR spectra, v(CC) generally occurs between 1590 and 1660 cm-1 in their Raman spectra, strong v(CC) bands are found at 1590 and 1594 cm-1 for... 

Mononuclear Propadienylidene Complexes Some Structural Parameters 1... 

Binuclear Vinylidene and Propadienylidene Complexes Some Structural Parameters1... 

Figure 2 summarizes the structural parameters of the vinylidene or propadienylidene ligands when bonded to various metal clusters. It can be seen that the C—C double bond is lengthened to ca. 1.4 A as a result of the interaction with the cluster the a-carbon bridges three or four metal atoms, with M-C distances consistent with normal single bonds. 

Vinylidene and Propadienylidene (Allenylidene) Metal Complexes, 22, 59 X-Ray Structural Analysis of Organolithium Compounds, 24, 353 Ziegler-Natta Catalysis, 18, 99... 

Propadienyliden)-triphenyl-E19d, 652 (Eliminierung) Thioketenyltden-triphenyl- El9d, 652 (Eliminuer.)... 

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