Vinylidene complexes structures

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

Reaction of the carbonyl complex 26 with the mercury diazomethane 27 gives the highly reactive 17e intermediate carbyne complex 28 which dimerizes to form the / -biscarbyne complex 30. In this case, the intermediate terminal carbyne complex 28 has been trapped by reaction with the mercury diazomethane 29 to form the cyclic vinylidene complex 31. 31 was also characterized by a single crystal X-ray structure analysis. 

Beside the spectroscopic evidence, the type A configuration is confirmed also by an X-ray structure determination of the vinylidene complex [Ru(bdmpza)Cl(=C=CHTol)(PPh3)] (33b) (Fig. 23). 

Obviously, the first intermediates in the syntheses with terminal alkynols are the vinylidene complexes [Ru(bdmpza)Cl(=C= CH(CH2) +iOH)(PPhg)] (n = 1, 2), which then react further via an intramolecular addition of the alcohol functionality to the a-carbon (Scheme 22), although in none of our experiments we were able to observe or isolate any intermediate vinylidene complexes. The subsequent intramolecular ring closure provides the cyclic carbene complexes with a five-membered ring in case of the reaction with but-3-yn-l-ol and with a six-membered ring in case of pent-4-yn-l-ol. For both products type A and type B isomers 35a-I/35a-II and 35b-I/ 35b-II are observed (Scheme 22, Fig. 22). The molecular structure shows a type A isomer 35b-I with the carbene ligand and the triphenylphosphine ligand in the two trans positions to the pyrazoles and was obtained from an X-ray structure determination (Fig. 25). 

There is not sufficient space to discuss all vinylidene complexes which have been reported, for example over 200 crystal structures are listed in the CCDC. Consequently, this article largely concentrates on the chemistry of metal vinylidene complexes which has been described since 1995. Vinylidene complexes are generally available for the metals of Groups 4—9, with several reactions of Group 10 alkynyls being supposed to proceed via intermediate vinylidenes. However, few of the latter compounds have yet been isolated. This chapter contains a summary of various preparative methods available, followed by a survey of stoichiometric reactions of vinylidene-metal complexes. A short section covers several non-catalytic reactions which are considered to proceed via vinylidene complexes. The latter, however, have been neither isolated nor detected under the prevailing conditions. 

The number of known, isolated and characterized complexes depends strongly on the length of the chain and drastically decreases with the number of carbon atoms in the chain. A great number of vinylidene complexes of many metals, with different terminal substituents R and various co-ligands have been synthesized and the reactivity has been studied extensively. At present, the solid-state structure of more than 230 vinylidene complexes has been determined by X-ray structure analyses. The number of isolated allenylidene complexes is somewhat smaller. 

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 ... 

In this chapter, we first analyzed the electronic structures of metal vinylidene and allenylidene complexes. The electronic structures allow us to understand the reactivities of these complexes. For metal vinylidene complexes of the Fischer-type, nucleophilic attack usually occurs at the a-carbon and electrophilic attack at the P-carbon. For the corresponding metal allenylidenes, electrophilic attack occurs at the P-carbon and/or the metal center. Then we briefly reviewed the theoretical study of the barriers ofrotation ofvinylidene ligands in various flve-coordinate complexes M (X) C1(=C=CHR)L2 (M = Os, Ru L = phosphine). The study showed that 7t-acceptor ligands (X), electron-withdrawing substituents and lighter metals gave smaller barriers. 

In this chapter, we summarized the theoretical studies carried out on metal vinylidene complexes. Special emphasis was placed on aspects of their electronic structures, reactivities and their roles in organic reactions. Theoretical studies on the related metal allenylidene complexes have been quite limited. More theoretical studies on various aspects of these complexes, particularly on their metathesis reactivities, are clearly necessary. 

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) ... 

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... 

As discussed earlier, the chemistry of the vinylidene complexes is influenced by steric constraints imposed by the flanking phosphine groups. The steric congestion about the ruthenium center has an even more pronounced effect on reactivity at Ca in carbene complexes. The crystal structure of complex 96 (61) provides an excellent example of the pro-... 

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. 

Quite recently, a platinum(II) vinylidene complex was reported. Its synthesis was accomplished by reaction of Pt(PPh3)2(CH3)(CCR) with triflic acid and tetrafluoroboric acid, respectively, to give the resulting cationic Pt(II) complex. (CH3)(PPh3)2Pt(=C= CHR)]+ X- (X = BF4-. Cp3S03-). ° No data on the catalytic activity of this complex, which has not been isolated but whose structure has been proven by NMR experiments, have been reported so far. 

Fig. 7. Molecular structures of (a) the vinylidene complex 587 (hydrogen atoms omitted) and (b) the iridabenzene 587 (Tp hydrogen atoms omitted).

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