Metal-alkynyl complexes bonding

The chemistry of bis(alkynyl) metal complexes, including the titanium derivatives, focusing on the synthesis, chemical behavior, structure, and bonding of different type of mononuclear and heterometallic molecules has been discussed in a review.1280 A review focusing the preparation and the reactivity of compounds of the type Cp 2Ti-(ct-C=CR)2 (Cp = Cp, Gp ) has appeared which summarizes special aspects of C-C coupling and cleavage processes in organic reactions.1281... 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

Etherification using a metal vinylidene has also been combined with G-G bond formation through the reaction of an alkynyl tungsten complex with benzaldehyde (Scheme 14). The addition of an internal alcohol to the incipient /3,/Udialkylvinylidene that is generated leads to dehydration and the formation of a Fischer-type alkylidene complex. Further reactions of this carbene with a range of nucleophiles have provided access to various furan derivatives.374,375... 

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]. 

Scheme 4.5 shows several possible pathways from r -acetylene metal complexes RE to metal vinylidenes PR. In the first pathway (al + a2), metal vinylidenes PR can be obtained from an intermediate (INI) with a 1,2 hydrogen shift from C to Cp. The second pathway (bl + b2) is through an intermediate (IN2) with an r agostic interaction between the metal center and one C—H bond, which undergoes a 1,2 hydrogen shift to PR. The third pathway (bl + b3 + b4) also starts from IN2 but then goes into another intermediate, the hydrido-alkynyl IN3, which leads to PR with a 1,3 hydrogen shift from the metal center to Cp. 

Dipolar cycloaddition reactions are most commonly applied for the synthesis of five-membered heterocyclic compounds.86 87 [3+2] cycloaddition reactions of transition-metal propargyl complexes have been reviewed.88 Addition of diazomethane to carbene complexes (CO)5Cr= C(OEt)R results in cleavage of the M = C bond with formation of enol ethers H2C = C(OEt)R,3 89 but (l-alkynyl)carbene complexes undergo 1,3-dipolar cycloaddition reactions at the M = C as well as at the C=C bond. Compound lb (M = W, R = Ph) affords a mixture of pyrazole derivatives 61 and 62 with 1 eq diazomethane,90 but compound 62 is obtained as sole... 

Because of the highly unsaturated character of (l-alkynyl)carbene complexes 1, a variety of metal -complexes can be derived from coordination of the M=C as well as of the C=C bonds. 

One of the characteristic features of the metal-bis-acetylide complexes in chemical reactions is that they undergo a ligand exchange reaction with metal dihalides in amines in the presence of a cuprous halide catalyst to produce a monoalkynyl-metal-monohalide complex (Eq. IS) which results from selective cleavage of the metal-carbon bond weakened by the /ram-alkynyl group in the bis-acetylide... 

AUcynylchalcogenato ruthenium complexes react with zirconocene to give rise to heterobimetallic early-late dissymmetrically bridged complexes of family (76) (equation 39). In those specific complexes, the two metal centers are linked by /u.-chalcogenido and /x-a, jr-alkynyl moieties. The acetylenic bridge is unsymmetrical because the terminal carbon of alkynyl is ct-bonded to ruthenium, while zirconium interacts with both alkynyl carbons in a side-on fashion. 

Work in our laboratory directed toward developing a class of transition-metal complexes that are structurally and electronically analogous to conjugated organic compounds and polymers led to the synthesis and spectroscopic study of alkynyl-substituted, quadruply metal-metal bonded compounds of the type M2(C=CR)4(PMe3)4 (6 M = Mo, W R =... 

Metal-Carbon Bond Lengths for Analogous Metal-Alkyl and Metal-Alkynyl Complexes... 

TT-backbonding, yet its CC-Si bond length [1.812(2) A] is not significantly shorter than that of the mean distance for all trimethylsilylethynyl metal complexes. Despite these uncertainties, the CC-Si bond distance bears close examination in future studies, particularly in conjunction with Si-NMR chemical shifts and MCC-Si nuclear spin-spin coupling constants, which have been reported to correlate with the extent of C-Si rr-bonding (184), because of the preliminary indication that it is shorter for metal-alkynyl complexes than in organic alkynes. 

---