Hydrido carbene complex

Section III.C A Hydrido(methyl)carbene Complex of Platinum(IV) (223) and Methyl(hydrido)platinum(IV) Complexes with Flexible Tridentate Nitrogen-Donor Ligands (224) are structurally related to the system shown in Scheme 13 and give additional information on how steric and electronic factors influence the stability of platinum(IV) methyl hydrides. 

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]. 

Recently, the oxidative addition of C2-S bond to Pd has been described. Methyl levamisolium triflate reacts with [Pd(dba)2] to give the cationic palladium complex 35 bearing a chiral bidentate imidazolidin-2-ylidene ligand [120]. The oxidative addition of the levamisolium cation to triruthenium or triosmium carbonyl compounds proceeds also readily to yield the carbene complexes [121], The oxidative addition of imidazolium salts is not limited to or d transition metals but has also been observed in main group chemistry. The reaction of a 1,3-dimesitylimidazolium salt with an anionic gallium(I) heterocycle proceeds under formation of the gaUium(III) hydrido complex 36 (Fig. 12) [122]. 

Optimized reaction conditions call for the use of Wilkinson s catalyst in conjunction with the organocatalyst 2-amino-3-picoline (60) and a Br0nsted add. Jun and coworkers have demonstrated the effectiveness of this catalyst mixture for a number of reactions induding hydroacylation and C—H bond fundionalization [25]. Whereas, in most cases, the Lewis basic pyridyl nitrogen of the cocatalyst ads to dired the insertion of rhodium into a bond of interest, in this case the opposite is true - the pyridyl nitrogen direds the attack of cocatalyst onto an organorhodium spedes (Scheme 9.11). Hydroamination of the vinylidene complex 61 by 3-amino-2-picoline gives the chelated amino-carbene complex 62, which is in equilibrium with a-bound hydrido-rhodium tautomers 63 and 64. 

A Fischer-type dithiocarbene complex (447) which undergoes protonation at the carbene carbon was reported by Le Bozec et al (265). The reaction, which produces 448, proceeds via formation of a /rans-hydrido-carbene intermediate, isomerization to the cis isomer, and a metal to carbene carbon hydrogen shift. 

In at least one case, there is an example of ethene polymerization using a Ta-carbene complex in which there is strong evidence for a metathesis-based mechanism. See H. W. Turner and R. R. Schrock, J. Am. Chem. Soc., 1982,104, 2331, a paper that describes oligomerization of up to 35 ethene units in the presence of Ta[=CH(f-Bu)](H)(PMe3)3l2. It seems clear that although the Cossee mechanism is operative when polymerization occurs in the presence of Z-N-type catalysts, some polymerizations may involve metathesis, especially when hydrido-metal carbenes can form readily. 

A cationic hydrido-methyl complex, HPtCHs ", is formed upon the almost barrierless insertion of a Pt ion into a C-H bond of methane and then 1,2-migration of a hydrogen atom and subsequent elimination of H2 from the transition-metal center gives the platinum carbene cation PtCHi"" with an apparent rate constant of k - 8.2 x 10" cm molecule" s. The reaction of ground-state platinum atoms produced by the photodissociation of [PtMe3-(CsHiMe)] follows a termolecular mechanism [4c] ... 

Unexpected 2,1-addition of Ru-H bond across the C=C bonds of ethyl vinyl ether [61], vinyl acetate [62] and vinyl chloride [63] followed by a-hydrido, a-acetato and a-chloro eliminations, respectively, to give carbene complexes has been also known. 

The debate on the mechanism of polymerization, whether an insertion mechanism (Cossee-Arlman) [6], or a metathesis-type mechanism initiated by a-H elimination from the alkyl complex to give a hydrido-carbene intermediate (Green-Rooney) [108], was solved in favor of the former on the basis of the absence of isotope effect on the rates of insertion, and on the stereochemistry of alkene intramolecular insertion, when a-D alkyls were used in the cyclizafion reaction shown in Eq. 6.21 [109]. 

Anionic metal complexes may react with organic dihalogeno compounds to form carbene complexes. Similarly, metal carbenes may be obtained by reactions of hydrido complexes with compounds possessing active hydrogen. 

The cationic methylplatinum(ii) complex with a [B(C6H3(CF3)2-3,5)4] counteranion, and Et20 and TMEDA as ligands, is converted into the hydrido(carbene)platinum(ii) complex 911, via a Pt(iv) intermediate 910 formed by G-H activation of the Et20 ligand, and subsequent reductive elimination of methane and transfer of a hydrogen from diethyl ether ligand to Pt(ii) (Equation (168)). " ... 

Cr, Mo and W - The gas-phase ion chemistry of the chromium cycloheptatriene complexes [Cr(T -C7H8)(CO)3] was studied272 by Fourier Transform Ion Cyclotron Resonance Spectroscopy. The presence of a fragment ion [C5H6Cr]+ with the hydrido-ri -cyclopentadienyl structure was detected. Reaction of the same complex [Cr(Ti -C7H8)(CO)3] with the carbene complex [Cr(CO)5(=C(OMe)Me)] afforded273 an isomeric mixture of bicyclo[4.2.1]nonanone products. 

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