Vinylidene complexes mononuclear

Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding 7t-complexes. With terminal alkynes the corresponding 7t-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [128,130,132,133,547,556-569]. With mononuclear rj -alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-Fl bond to yield a hydrido alkynyl eomplex, followed by 1,3-hydrogen shift from the metal to Cn [570,571], or (b) eoneerted formation of the M-C bond and 1,2-shift of H to Cp [572]. 

Water-soluble mthenium vinyUdene and aUenylidene complexes were also synthetized in the reaction of [ RuCl2(TPPMS)2 2] and phenylacetylene or diphenylpropargyl alcohol [29]. The mononuclear Ru-vinylidene complex [RuCl2 C=C(H)Ph)(TPPMS)2] and the dinuclear Ru-aUylidene derivative [ RuCl(p,-Cl)(C=C=CPh2)(TPPMS)2 2] both catalyzed the cross-olefin metathesis of cyclopentene with methyl acrylate to give polyunsaturated esters under mild conditions (Scheme 7.10). 

Preparation and Stoichiometric Reactivity of Mononuclear Metal Vinylidene Complexes... 

Species (A) and (B) constitute the main class of unsaturated carbenes and play important roles as reactive intermediates due to the very electron-deficient carbon Cl [1]. Once they are coordinated with an electron-rich transition metal, metal vinylidene (C) and allenylidene (D) complexes are formed (Scheme 4.1). Since the first example of mononuclear vinylidene complexes was reported by King and Saran in 1972 [2] and isolated and structurally characterized by Ibers and Kirchner in 1974 [3], transition metal vinylidene and allenylidene complexes have attracted considerable interest because of their role in carbon-heteroatom and carbon-carbon bond-forming reactions as well as alkene and enyne metathesis [4]. Over the last three decades, many reviews [4—18] have been contributed on various aspects of the chemistry of metal vinylidene and allenylidene complexes. A number of theoretical studies have also been carried out [19-43]. However, a review of the theoretical aspects of the metal vinylidene and allenylidene complexes is very limited [44]. This chapter will cover theoretical aspects of metal vinylidene and allenylidene complexes. The following aspects vdll be reviewed ... 

Reactions of mononuclear vinylidene complexes with other reactive metal complexes to give binuclear //-vinylidene complexes have been described above. Addition of Fe2(CO)c, to Mn(C=CHPh)(CO)2(i/-C5H5) also gives 31, by addition of a CO group to the a-carbon structural data are consistent with the delocalized formulation (31b), with its obvious resemblances to trimethylenemethane (60) ... 

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). 

BONDING AND STRUCTURE IN MONONUCLEAR AND BINUCLEAR VINYLIDENE COMPLEXES... 

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).

Mononuclear Vinylidene Complexes Some Structural Parameters 1... 

Several vinylidene complexes containing terminal and bridging vinyl-idene ligands are known, but the title complex and simple derivatives are the only ones whose photochemistry has been studied. Caulton and co-workers (186) observed that irradiation of 153 in the presence of cyclohexene gave disruption of the dimer and formation of the mononuclear vinylidene and cyclohexane complexes 154 and 155 [Eq. (147)], along... 

Photochemical desulfurization of thiocarbonates occurs with [Fe(CO)j] to give mononuclear as well as trinuclear (type 8) carbene complexes (24). A similar reaction affords desulfurization of diphenylcyclopropenethione (22). The thioketene cluster 13 reacts quantitatively, but in a more complicated way, with a thioketene to form a dinuclear vinylidene complex (14). 

The homobimetallic, ethylene-ruthenium complex 15, which contains three chloro bridges, was readily obtained from the reaction of [RuCl2(/ -cymene)]2 with 1 atm of ethylene [34]. In 2009, Demonceau and Delaude [34] showed that complex 15 could be a useful precursor to allow subsequent access to the diruthenium vinylidene complex 16, allenylidene complex 17, and indenylidene complex 18 (Scheme 14.8). Upon reaction with propargylic alcohol, complex 15 afforded vinylidene complex 16, which converted into the allenylidene complex 17 in the presence of molecular sieves [34]. As shown in the acid-promoted intramolecular rearrangement of mononuclear ruthenium allenylidene complexes [19, 20, 32], the addition of a stoichiometric amount of TsOH to complex 17 at -50 °C led to the indenylidene binuclear complex 18 [34]. Complex 18 has been well... 

Protonation of 124 using either HBF4 or NH4[PF6] gives the cationic vinylidene complexes, [Fe(=C=CHCH2CH2GN)(dppe)(Cp)]X (X = Bp4 or PFe). The dimeric complex, [(Gp)(dppe)[Cp(dppe)-Fe(C=CCH2GH2GN)Fe(dppe)Cp][PF6], formed by the reaction of 124 with [Fe(NCMe)(dppe)(Cp)][PF6] was unstable compared to its mononuclear analog. 

This route is similar to those syntheses starting from mononuclear complexes with a terminal vinylidene ligand (see Scheme 17).54,55 In both cases a thioketene complex is formed as an intermediate, which is then reduced. 

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