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... 

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... 

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)] ... 

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. ... 

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)]. 

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. 

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... 

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. 

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. 

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. 

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],... 

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