Carbyne ligand

Byers, P.K, Carr, N. and Stone, F.G.A. (1990) Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 106. Synthesis and reactions of the alkylidyne complexes [M ( CR)(CO)2 (C6F5)AuC(pz)3 j (M = W or Mo, R — alkyl or aryl, pz — pyrazol-l-yl) crystal structure of pjC PtAu(C6F5)( l3-CMe)(CO)2(PMe2Ph)2 (C6F5)AuC(pz)3 ]. Journal of the Chemical Society, Dalton Transactions, (12), 3701—3708. 

The implication is that the stronger the rr-donating ability of the substituent, the weaker the rr-accepting ability of the carbyne ligand. 

The change to a silicon-based substituent group, e.g., SiMe3, has the opposite effect. The introduction of two more orbitals of 7r-symmetry appropriate for bonding stabilizes the metal-carbon interaction and increases the percentage electron density of the 7r-orbital on the carbyne ligand (28). 

The introduction of a new carbyne ligand to the metal in a single step. 

Isocyanides also display over a wide range of Pi values (Table 2), from benzoyl isocyanide Pi = +0.20 V) [15] with a net electron-acceptance similar to that of the carbyne ligands, down to the metallo-cyanide adduct C=N-PdCl2(PPh3) Pi = -0.61 V) [27]. Between these limiting cases, one finds the following types ordered according to their net electron-acceptance (Tables 2... 

The carbyne ligand may be viewed as a three-electron donor, similar to the nitrosyl ligand, with a pair of electrons in an sp orbital and a single electron in a p orbital. Donation of the sp electrons and pairing the p electron with one from the metal atom gives a a bond and a 7r bond, respectively. The second rbond results from donation of an electron pair from the metal atom to the empty p orbital of the ligand. 

It also proved possible to put alkyl, carbene, and carbyne ligands into the same molecule.72... 

Fig. 1538 Energy barriers (in kJ moT1) for three separate types of rotational motion involving a bridging vinyl carbyne ligand. The values were obtained by line shape analysis of variable temperature proton NMR xctra. [From Casey. C. P Konings, M. S. Murder.

Heterodinuclear iron compounds, with /r-carbyne ligands,... 

The ligand perceptions presented in Fig. 3 concern electronic characteristics. Donors, neutral or anionic ligands, 7t-donor neutral ligands, and so on are perceived. Carbene and carbyne ligands are considered as donor neutral ligands, and a distinction between Schrock -type and Fischer -type car-benes is made (9). 

It was also of interest to us how changes in the organic residue of the carbyne ligand influence the stability and the behavior of carbyne complexes. Hence, we treated with boron tribromide a series of pentacarbonyl-[methoxy (aryl) carbene tungsten (0) complexes which were substituted at... 

Simultaneously, we have been occupied with the question as to how the carbyne ligand behaves when it is split off from the metal. Analogous to the carbene complexes, in the absence of a suitable reaction partner, one also observes dimerization, in this case to alkynes (100). The conditions for the removal are very mild. In nonpolar solvents, diphenylacetylene or dimethylacetylene are accessible in this way at 30°C. One arrives at the same result when the solid methylcarbync complex is heated to 50°C ... 

Thus, the way appears to be open to make carbyne complexes for use in the syntheses of organic compounds. Since to our knowledge there is no specific and selective carbyne source available for preparative purposes, presumably there arises here a wide field of interesting possibilities of application, especially because of the mild conditions required to transfer the carbyne ligand. 

H. P. Kim, and R. J. Angelici, Transition Metal Complexes with Terminal Carbyne Ligands, Adv. Organomet. Chem. 27, 51-111 (1987). 

Figure 1.12 suggests that for carbonyl complexes the HOMO is localized primarily on the metal centre, with only a modest contribution from oxygen orbitals. Thus by far the majority of reactions of metal carbonyls with electrophiles involve direct attack at the metal, with the carbonyl serving as a spectator ligand. If, however, the metal centre is (i) particularly electron rich and (ii) sterically shielded and the electrophile is hard (in the HSAB sense) and also sterically encumbered, then attack may occur at the oxygen. Thiocarbonyls (LM-CS) are stronger 71-acids than CO and the sulfur is both softer and more nucleophilic. Thus electrophilic attack at the sulfur of thiocarbonyls is more common if the metal centre is electron rich (vcs < 1200 cm-1). Similarly, coordinated isocyanides (CNR) are more prone to attack by electrophiles at nitrogen. This is noteworthy in the sense that free isocyanides are attacked by electrophiles at carbon (Figure 3.19). The resulting carbyne ligands will be discussed in Chapter 5.

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