Alkynes, complexes

Alkynes have normally been considered as ligands that bind to low-valent metals. Several recent cases of binding to high-valent centers have changed 

Suggest reasons why Ti(CH2Ph)4 does not form a stable CO adduct. 

Why do you think V only gives VR4 as the highest-oxidation-state alkyl, but Ta can give TaRs  

What mechanism is likely for Eq. 15.3, and would 15.1 and 15.2 be likely to give the same type of reaction  

The ethylenes in Mo(C2H4)2(PR3)4 are mutually trans. What do you think the orientation of their C=C bonds would be with respect to one another (Draw this looking down the principal axis of the molecule.) 

Theoretical calculations on T)2-ethyne complexes of second row transition metals and their insertion products have been reported362. it was found that for most atoms the 7C-coordinated complexes are thermodynamically favoured, just as in the case of the corresponding ethene reaction. The barrier height for the ethyne reaction was found to increase significantly between niobium and molybdenum, associated with the absence of vacant d orbitals in the latter atom. 

Structural and bonding patterns in tetrahedral and pseuodo-tetrahedral complexes containing alkene, alkyne and alkylidyne ligands have been reviewed S and it has been demonstrated how the orientations of such ligands acts as a compass needle to reveal n-bonding effects in the complexes. 

The mode of reaction of the reduced titanocene species Cp2Ti with disubstituted butadiynes RCC-CCR was reported to depend strongly on the nature of the substituents R and R . For R = R = SiMea the starting butadiyne is split by titanocene to yield the dinuclear complex [ Cp2Ti(CCSiMea) 2] (43). For other symmetrically substituted (R = R = Ki or Bu ) or unsymmetrically substituted butadiynes (R = SiMea, R = Ph or R = SiMea, R = Bu ) the reaction was reported to proceed to give dinuclear complexes with a central 1,4-disubstituted ii-T (1-3),ti(2-4)-trans, trans-butadiene unit ( zig-zag-butadiyne ) between the two titanium centres. The latter compounds with (R = R = Bu ) and (R = SiMea, R = Ph) were characterised by X-ray diffraction. 

The reaction of [Cp2ZrCl2] with [l,2-C6H4(CCSiMea)2] and butyllithium at low temperature in THF afforded a zirconocene bis-alkyne complex, (44), which was characterised by X-ray diffraction. The bonding in (44) was represented by three alkyne to metal interactions, two of which involve donation from the alkyne s rt-systems to the metal and one of which is back-bonding from the metal to the 7c -system of the alkynes, so that the compound can best be represented as the superposition of the two degenerate resonance contributors shown in (45). 

CCSiMe3)( ii-T FTi2-CCPh)ZrCp2] respectively. The structure of each of the above products was established by X-ray diffraction. 

The three bonding interactions of metal-alkyne complexes. 

Many of the structural changes that occur upon bonding of an olefin to a metal also occur upon bonding of an alkyne to a metal. The acetylenic group is distorted toward the geometry of a cis olefin- when coordination occurs. The carbon-carbon bond becomes 

Coordination of an acetylene to a transition metal makes the acetylene stretching frequency infrared allowed and shifts the vibration to frequencies that are 150 to 450 cm below that of the Raman band of the free alkyne. Larger changes in the vibrational frequency wpuld be expected when the complex adopts the structure of a metallacyclopropene instead of a coordinated alkyne, but this prediction has not been carefully explored. Like alkene complexes, alk3me complexes can possess rotational barriers about the metal-acetylene axis, and these rotational barriers depend on the symmetry of the orbitals of the metal fragment. 

An equilibrium between ti - and ti -aldehyde complexes. Structural data are available.  

The resulting expansiveness of this field prevents a comprehensive review. It is our intention, therefore, to highlight some of the most important and recent developments in the chemistry of metal-alkyne complexes. We will largely limit our coverage to that chemistry which clearly involves the intervention of metal- i-bonded alkyne complexes. We thus exclude the chemistry of metal-acetylide derivatives and mention only briefly the burgeoning number of metal-catalyzed reactions for which alkyne complexes are only presumed intermediates. Prior reviews of metal-alkyne chemistry may be consulted for more complete coverage of the older literature [3]. 

Naked [Fe4] ions in the gas phase were shown by mass spectrometry to oligomerise ethyne to benzene via the cationic intermediate [Fe(C2H2)m] (m = 1-4)256. r was reported that the ethene ligand in [Fe(CO)2 P(OMe)3 2(Tl2-C2H4)] could be substituted by diphenylacetylene to form [Fe(CX))2 P(OMe)3 2(il -C2Ph2)]. The X-ray crystal structures of the alkene and alkyne complexes were determined257. Reaction of [CpFe P(OMe)3)2(NCMe)]+[PF6] with [PhCXH] 

The use of a cyclopentadienyl ligand containing a pendant phosphine arm allowed the isolation of the cobalt alkyne complexes [ 5o(ti5 i -C5H4CH2CH2l Bu 2)(Tl -PhCCR)] (R = H, Hi). In the case of ethyne, the alkyne complex could not be isedated and the vinylidene complex [ 50(115 I-C5H4CH2CH2PBu 2)(=C=CH2)1 was observed. 

Copper(I) alkyne complexes of the type [Cu(phen)(Ti2-RCCH)]+[C104] (R = H, Ph CO2EO have been prepared and structurally characterised d, closely related complexes of the type [Cu(bipy)( n2-alkyne)] [PF6] (alkyne = hex-3-yne, pent-l-yne) have also been reported Upon repeated recrystallisation it has been proposed that dimers of the type 

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Alkyne complexes are essentially similar to the alkenes (p. 932) and those of Pt particularly when the alkyne incorporates the <-butyl group, are the most stable. Ni alkyne complexes are less numerous and generally less stable but are of greater practical importance because of their role as intermediates in the cyclic oligomerization of alkynes, discovered by W. Reppe (see Panel). 

Initial step is the formation of a dicobalthexacarbonyl-alkyne complex 5 by reaction of alkyne 1 with dicobaltoctacarbonyl 4 with concomitant loss of two molecules of CO. Complex 5 has been shown to be an intermediate by independent synthesis. It is likely that complex 5 coordinates to the alkene 2. Insertion of carbon monoxide then leads to formation of a cyclopentenone complex 6, which decomposes into dicobalthexacarbonyl and cyclopentenone 3 ... 

Products 7a and 7c, with the substituent R a to the carbonyl group, are by far predominantly formed. This regioselectivity is a result of the preferential approach of the alkene 2 to the dicobalthexacarbonyl-alkyne complex 5 from the side opposite to the substituent R of the original alkyne. The actual incorporation of the alkene however is less selective with respect to the orientation of the olefinic substituent R, thus leading to a mixture of isomers 7a and 7c. 

B. Alkene and Alkyne Complexes 1. Mono-enes and Mono-ynes... 

The reaction of alkenes with alkenes or alkynes does not always produce an aromatic ring. An important variation of this reaction reacts dienes, diynes, or en-ynes with transition metals to form organometallic coordination complexes. In the presence of carbon monoxide, cyclopentenone derivatives are formed in what is known as the Pauson-Khand reaction The reaction involves (1) formation of a hexacarbonyldicobalt-alkyne complex and (2) decomposition of the complex in the presence of an alkene. A typical example Rhodium and tungsten ... 

Attempts to exploit the reaction of the dianion with alkyl halides to produce a c/.v-dialkyl complex by using 1,2- or 1,3-dihaloalkanes did not indeed give this result. The reaction of Ru(Por) " with 1,2-dibromoethane was sucessful, but the resulting metallacyclopropane product is better formulated as a /r-complex of ethene, and will be discussed below in the section on alkenc and alkyne complexes. The corresponding reaction of the diiinion with 1,3-dichloropropane gave no evidence for a metallacyclobutane. but instead free cyclopropane was detected by GC analysis and the porphyrin product was Ru(TTP)(THF)2. ... 

Melikyan GG, Nicholas KM (1995) The Chemistry of Metal-Alkyne Complexes. In Stang... 

The strong o-donor property of NHC ligands enhances the catalytic activity in [3+2] cycloaddition by promoting the activation of internal alkynes (i.e. 26), which proceeds by the formation of a ti-alkyne complex 25 (Scheme 5.7). 

Two S/P ligands derived from camphor, CamPHOS and MeCamPHOS were also developed by these authors for the diastereoselective coordination to alkyne-hexacarbonyldicobalt complexes (Scheme 10.68). These two ligands were converted in good yields into their borane-protected forms. The influence of the alkyne group (R) on their coordination to dicobalt-hexacarbonyl-alkyne complexes was evaluated. It was shown that MeCamPHOS ligand provided a... 

R = alkyl.502 (307) (R1 = R2 = Ph) reacts with alkynes to form a ketene alkyne complex, (308), which ultimately yields the five-coordinate Ir111 species (309).503... 

The copper-alkoxo unit, which is usually synthesized in situ, plays a significant role in metal-promoted transformations of organic substrates by copper(I). To determine the reaction form of the Cu-OPh unit, Floriani and co-workers structurally characterized four complexes (772) (pseudotetrahedral Cu-Cu 3.223 AT (773) (pseudotetrahedral), (774) ( anion linear coordination) and (775) (planar trigonal).57 Using 3,3,6,6-tetramethyl-l-thia-4-cycloheptyne as terminal ligand the structural characterization of a copper(I)-alkyne complex (776) (Cu-Cu 2.940 A) was reported.573... 

Went, Michael J., Synthesis and Reactions of Polynuclear Cobalt-Alkyne Complexes. 41 69... 

The alkyne complex (dtbpm)Pt(Me3SiC2SiMe3) is also formed, but is not a precursor to the C-Si activation product. 

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