Metal—carbon triple bonds nucleophiles, reactions with

Nucleophilic displacement at a terminal methylidyne ligand, with regeneration of a metal - carbon triple bond in 199 (E = S or Se, R = Me, Ph, or P-NO2QH4) to form a new methylidyne ligand, occurs in the reaction of 198 [L = HB(3,5-Mc2—C3HN2)3] with thiolate or selenolate anions under phase-transfer conditions 149). 

The reagent [MejSSMe] is capable of transferring a [SMe] group to metal-carbon triple bonds [see also Eq. (118)] 142). The reaction may be viewed as a direct electrophilic attack at the MC n bond. With complex 128 the dithiocarbene complex 129 is obtained [Eq. (112)]. Complex 129 reacts with nucleophiles (H , CHj", and PMe3), which add to the carbene carbon to give stable adducts [Eq. (113)]. The carbonylmetallate anions... 

Reactivity modes of the poly(pyrazolyl)borate alkylidyne complexes follow a number of recognised routes for transition metal complexes containing metal-carbon triple bonds, including ligand substitution or redox reactions at the transition metal centre, insertion of a molecule into the metal-carbon triple bond, and electrophilic or nucleophilic attack at the alkylidyne carbon, C. Cationic alkylidyne complexes generally react with nucleophiles at the alkylidyne carbon, whereas neutral alkylidyne complexes can react at either the metal centre or the alkylidyne carbon. Substantive work has been devoted to neutral and cationic alkylidyne complexes bearing heteroatom substituents. Differences between the chemistry of the various Tp complexes have previously been rationalised largely on the basis of steric effects. 

Nucleophiles react with carbyne complexes to promote (or trap) carbyne-carbonyl coupling products, attack the metal-carbon triple bond, or displace a substituent on the carbyne carbon. Complexes of the form Tp M( = CR)(CO)2 are coordinatively saturated and in the case of Tp = Tp, the metal also enjoys a substantial degree of steric protection. Accordingly, the reaction of these complexes with nucleophiles does not, in general, involve attack at the metal but rather at a coligand. Attempted synthesis of Tp W( = CMe)(CO)2 via reaction of MeLi with the... 

Also alkynylcarbene complexes can react as Michael acceptors with nucleophiles, forming 1,3-dien-l-ylcarbene complexes (Figure 2.17). Both carbon nucleophiles, such as, e.g., enamines [246-249], and non-carbon nucleophiles, such as imidates [250], amines [64,131,251], aliphatic alcohols [48,79,252], phenols [252], and thiols [252] can add to the C-C triple bond of alkynylcarbene complexes. Further reactions of the C-C triple bond of alkynylcarbene complexes include 1,3-dipolar [253,254], Diels-Alder [64,234,238,255-258] and [2 -i- 2] cycloadditions [259 -261], intramolecular Pauson-Khand reactions [43,262], and C-metallation of ethynylcarbene complexes [263]. 

Carbynes, (RCj), are 2e ligands and their carbon atoms have sp hybridization (two p orbitals are not hybridized). Therefore, in carbyne complexes L M —CR, two molecular orbitals d -p are formed. Thus, formally, the metal-carbon bond is triple. Calculations of electronic structures of [(OC)5Cr = CH] and [(OC)4ClCr=CH] show that the carbyne carbon charge is negative. Because in cationic carbyne complexes the carbon atom reacts with nucleophiles, this carbon s reactivity is not controlled by the charge, contrary to these calculations. The contradictions between calculations and experiments may be explained " by means of the frontier orbital theory (reactions controlled by frontier orbitals). Nucleophiles react with the atom of a molecule on which the LUMO is located, while electrophiles react with the atom of a molecule on which the HOMO is mainly localized. In the case of [(OC)5Cr = CH], the maximal coefficient in LUMO is at the carbyne carbon atom, and therefore it reacts with the nucleophile despite the fact that it is negative. 

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