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Hypercoordinated hydrogens

Recent studies on hydrazide-based hypercoordinate silicon complexes demonstrated the unusual flexibility of these compounds their tendency to reversibly transform between penta- and hexacoordinate compounds [1, 2] on the one hand and to irreversibly rearrange to more stable complexes [3] on the other. Thus, neutral hexacoordinate bis-chelate complexes (1) undergo reversible ionization in solution (Eq. 1), which is strongly dependent on a variety of factors temperature (ionization is enhanced at low temperatures), solvent (ionization takes place in hydrogen-bond donor solvents such as CHCI3, CH2CI2, and CHFCI2), the nature of the anion, the... [Pg.297]

The hydrogen-bridged carbocation (B) contains a two-coordinate hydrogen atom. Hypercoordination— which includes two-coordination for hydrogen and at least five-coordination for carbon—is generally... [Pg.34]

The many reactions that involve insertion of alkenes or alkynes into metal-carbon or metal-hydrogen bonds provide further examples of hypercoordination of carbon atoms during reactions. For example, an alkene may coordinate to the coordinatively unsaturated metal atom of a metal hydride complex prior to inserting into the metal-hydrogen bond [Eq. (1.9)] ... [Pg.30]

Although their hydrogen atoms were not located, their relatively short metal-metal distances and acute M-C-M angles at the hypercoordinated carbon atoms show the metal-carbon bonding to resemble that in A Mce discussed previously. This resemblance to the aluminum system is underlined by the structure of the mixed metal methyl Mg(AlMe4)2 (28), also established by an X-ray study. ... [Pg.51]

The cyclopentyl cation (34) shows a single peak in the ll NMR spectrum of 5 H 4.75 even at -150°C. In the NMR spectrum/ a 10-line multiplet centered around 95,4ppm with. /< h = 28.5Hz was observed.This is in excellent agreement with values calculated for simple alkyl cations and cyclopentane and supports the complete hydrogen equilibration by rapid 1,2-shifts (through the 35 hypercoordinate intermediate or transition state) [Eq. (6.24)]. [Pg.317]

Carbon-Hydrogen Bond Insertion In the early 1960s the activation of alkanes by metal systems was realized from the related development of oxidative addition reactions " " in which low-valent metal complexes inserted into carbon-heteroatom, silicon-hydrogen, and hydrogen-hydrogen bonds. The direct oxidative addition of metals into C-H bonds was found in the cyclometallation reaction [Eq. (6.61)].The reverse process of oxidative addition is called reductive elimination, which involves the same hypercoordinate carbon species. [Pg.342]

In most cases, a large linewidth for the Ge signal can be associated with hypercoordination between the germanium atom and, in their case, the oxygen donor. A contribution from hydrogen bonding is likely, but this cannot be the sole reason for... [Pg.172]

Molecules that seem to violate the octet (or duplet) rule are hypercoordinate, because the central atom is bonded to an unconventionally high number of ligands . The simplest hypercoordinate, neutral, even-electron compounds of the first full row can be conceptually obtained by adding a hydrogen molecule to the normal hydrides (Fig. 4.1). Let us examine a related molecule which has substantially weathered in silico the tests that have been applied to its putative existence, namely nitrogen pentafluoride. [Pg.53]

Fig. 4.1 The simplest neutral, even-electron hypercoordinate first-row molecules can be considered to result from combining a normal hydride with a hydrogen molecule... Fig. 4.1 The simplest neutral, even-electron hypercoordinate first-row molecules can be considered to result from combining a normal hydride with a hydrogen molecule...
A carbon or hydrogen atom is hypercoordinated if the number of attached atoms exceeds four or one respectively. [Pg.80]

The hypercoordinated (9-M-(n 4- 1)) radicals of the general formula (MH + i) may or may not be stable, with respect to the dissociation to a hydrogen atom and the normal-coordinated MH molecule, depending on the nature of the M atom. Three cases may be observed, as shown in 38-40. In 38 the hypercoordinated species is more stable than the normal-coordinated constituents, while in 39 the species is metastable, and unstable in 40. [Pg.310]

In these compounds the Si-H bcnid is likely to act as a o-donor to the transition metal (agostic interaction), or the Si-bound hydrogen atoms act as o-donors towards the transition metal, thus constraining the Si atom in close proximity to the transition metal even without o-donor action from the latter to the Si atom. Besides, there are review articles on this topic, as mentioned in the introduction [36-40]. Hence, compounds such as 158 and 159 will not be discussed in detail here [327, 328]. Also, the same applies to compounds with Si transition metal bond which only achieve Si-hypercoordination by the presence of metal-bonded hydrogen atoms in closer proximity of the Si atom, while the rest of the Si coordination sphere is almost tetrahedral, thus less indicative of H Si donor action (e.g., 160) [329],... [Pg.76]


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See also in sourсe #XX -- [ Pg.5 ]




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Hypercoordination

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