Binuclear Vinylidene Complexes

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).

One of several complexes formed by photochemically reacting CpRe(CO>3 and PhC CH in THF, was the binuclear complex Cp-(CO)2Re M-T T)--(=C=C(Ph)C(Ph)==CH2) ReCp(CO)2 (65) (129). The formation of 65 involves a photoinduced coupling of two acetylene units. The structure of 65 reveals that the vinyl substituent of a vinylidene complex is Tj -bonded to a second rhenium center. 

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... 

The acylate anion from Mn(CO)3(j/-C5H5) and LiMe reacts similarly reaction with l,8-bis(dimethylamino)naphthalene (proton sponge) affords the vinylidene, although in this reaction, it is the binuclear complex which is isolated as the final product (38) ... 

Reactions between halogenoalkynes and the lactone complexes (34) afford a series of binuclear orange-red //-vinylidene derivatives (35) (62) ... 

Attempts to prepare the dimethyl analog from HC=CCMe2(OH) led instead to the binuclear vinylidene-carbene complex (49) (72) ... 

The major orbital interactions of the /i-vinylidene ligand in binuclear complexes are with (i) an unperturbed nxy orbital of the Rh2 fragment, (ii) a bonding linear combination of the Rh2 n y and C p orbitals, and (iii) a bonding linear combination of the Rh2 a orbitals. The small rotation (0° -14°) which is often found between the CR2 and the CM2 planes serves to optimize these orbital overlaps. 

Binuclear Vinylidene and Propadienylidene Complexes Some Structural Parameters1... 

The ability of the binuclear complex [Cp RuCl(p2-SR)2RuCl(Cp )] to generate cationic allenylidene complexes by activation of terminal prop-2-ynols in the presence of NH4BF4 as a chloride abstractor opens the way to a variety of catalytic transformations of propargylic alcohols involving nucleophilic addition at the Cy atom of the ruthenium allenylidene intermediate (Scheme 19). This leads to the formation of a functional ruthenium vinylidene species which tau-tomerizes into an -coordinated alkyne that is removed from the ruthenium centre in the presence of the substrate. 

In contrast to oxygen, sulfur has been far less widely used as a nucleophile for addition to ruthenium vinylidenes. The formation of vinyl thioethers from the addition of thiols to aUcynes catalyzed by binuclear ruthenium complexes Cp Ru (p-SR)2RuCp (R=Et, i-Pr, f-Bu) and related complex Cp Ru(p -C6F5)(p-S) (p-SC6F5)RuCp has been described [112]. 

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