Carbon atom high-coordinate

So far the vacant surface site has been indicated as a simple. On an atomic level the adsorption site is an ensemble of surface atoms, as shown in Figure 5.18 for adsorbed carbon atoms C coordinates to a site of four surface-metal atoms. In general, adsorbed molecules or atoms can adsorb to sites with different coordination numbers. Atoms usually prefer high coordination. Therefore, coadsorption of... 

The electrophilic carbene carbon atom of Fischer carbene complexes is usually stabilised through 7i-donation of an alkoxy or amino substituent. This type of electronic stabilisation renders carbene complexes thermostable nevertheless, they have to be stored and handled under inert gas in order to avoid oxidative decomposition. In a typical benzannulation protocol, the carbene complex is reacted with a 10% excess of the alkyne at a temperature between 45 and 60 °C in an ethereal solvent. On the other hand, the non-stabilised and highly electrophilic diphenylcarbene pentacarbonylchromium complex needs to be stored and handled at temperatures below -20 °C, which allows one to carry out benzannulation reactions at room temperature [34]. Recently, the first syntheses of tricyclic carbene complexes derived from diazo precursors have been performed and applied to benzannulation [35a,b]. The reaction of the non-planar dibenzocycloheptenylidene complex 28 with 1-hexyne afforded the Cr(CO)3-coordinated tetracyclic benzannulation product 29 in a completely regio- and diastereoselective way [35c] (Scheme 18). 

The crystal structures of many organolithium compounds have been determined.44 Phenyllithium has been crystallized as an ether solvate. The structure is tetrameric with lithium and carbon atoms at alternating corners of a highly distorted cube. The lithium atoms form a tetrahedron and the carbons are associated with the faces of the tetrahedron. Each carbon is 2.33 A from the three neighboring lithium atoms and an ether molecule is coordinated to each lithium atom. Figures 7.2a and b show, respectively, the Li-C cluster and the complete array of atoms, except for hydrogen 45 Section 6.2 of Part A provides additional information on the structure of organolithium compounds. 

An interesting example of t 3 coordination of a boratabenzene to a transition metal has been observed for a Zr(IV)-boratanaphthalene complex (Scheme 20).35 Thus, treatment of the illustrated 1-boratanaphthalene with Cp+ZrCl3 furnishes an adduct in which the metal-carbon distance for the 3, 4, and 5 positions of the heterocycle (2.53-2.56 A) is significantly shorter than for the 2 and 6 positions (2.77-2.81 A). It is suggested that this distorted bonding mode is the consequence of the high electron demand of Zr(IV), which prefers coordination to the most electron-rich carbon atoms. ... 

Carbon nanotubes, as graphene and graphite, are highly ordered carbon phases. However, a separate line can be drawn for historical development of disordered carbon phases among them is an amorphous carbon (am-C). In it, strong bonding between carbons did not allow for completely chaotic distribution of carbon atoms in solid-state phase. Instead, amorphous carbon exhibits random distribution of three possible coordinations of carbon atoms in a planar sp, tetrahedral sp and... 

The deshielding of the vinylic carbon atoms in the norbornyl cations 13, 55-57, 62 and 77 compared to the precursor compounds was taken as evidence for the intramolecular coordination of the C=C double bond to the electron-deficient element atom." ° This high-frequency shift, is largest for the silanorbornyl... 

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