Hydride bond strength

Open-chain alkanes, alkyl halide reduction, 29-31 Organosilicon hydrides bond strengths, 5-6 hypervalent silicon species, 9-11 ionic hydrogenation, 5 trivalent silicon species, 7-9 Orthoesters, reduction of, 97-99 Oxime reduction, 102... 

The most striking vertical trend concerns the metal-hydride bond strengths, as summarized in Table 4.55. We define the first M—H bond strength (A mh(1)) for each species as the energy needed to break the first M—H bond ... 

Insertion into element-hydrogen bonds tend to be less favored thermodynamically than insertions into other bonds (e.g., element-carbon). This is often attributed to the high element-hydride bond strength, which is lost upon insertion. Since the insertion reaction is also entropically disfavored, the reverse deinsertion of the unsaturated moiety to generate an element-hydride bond can be thermodynamically favored. When the hydride exists in the P position of the inserted product, this process is commonly referred to as /S-hydride elimination. Nevertheless, there are many examples of insertions into element-hydride bonds that generate stable compounds, and when this insertion reaction is an uphill process, chelation to the element or subsequent chemistry (i.e., catalytic cycles) can be employed to facilitate the initial insertion step. 

Hydrides of Nonmetallic elements form molecular hydrides. Bond strengths and stabilities decline down nonmetals each group. Some have Bronsted acidic and basic properties. 

However, the significant key difference for rhodium arises from the chemistry of the Rh(ll) dimer, [Rh(Por)]2, which exhibits a relatively low Rh—Rh bond strength. It undergoes homolytic dissociation and exists in equilibrium with the monomer, Rh(Por)- (Eq, (15)). The rhodium dimer can also exist in equilibrium with the hydride Rh(Por)H (Eq. (16)), and thus the hydride complex can exhibit the chemistry of the dimer, driven by formation of the Rh(Por)- monomer formed as in Eqs. (15) and (16). 

According to Baeyer, the first member of the series of strained carbon hydride ring compounds is ethylene (n=2). Our last basic statement connects ring strain with ir-bond strength. 

Consideration of the nature of the Si-H bond provides insight into the chemical behavior of organosilicon hydrides. Comparison of the bond strengths as... 

Because of the lower metal-carbon and metal-hydrogen bond strength, organolead hydrides are particularly unstable species and represent the least stable of those of the group 14 elements. Triorganolead hydrides are obtained at low temperatures by reduction of the halides with LiAlH4 (equation 45), but they decompose at 0 °C. 

As an improved metric of M—M quintuple-bond strength, let us instead consider the energetics of hydrogenation of HMMH to form two MH6 metal hydrides,... 

On the basis of known shapes of hydride bonds and antibond NBOs (Section 3.2.6) and their dependence on the relative electronegativity of A and H, we can predict certain geometrical, energetic, and dielectric features of B HA hydrogen bonding, all related to the strength of nB— ctah charge-transfer delocalization. 

Table I includes the relative bond dissociation enthalpies obtained for some group 14 hydrides by photoacoustic calorimetry,7 10 The data demonstrate that, for the trialkyl-substituted series, the bond strengths decrease by 6.5 and 16.5 kcal/mol on going from silane to germane and to stannane, respectively. The silicon-hydrogen bonds can be dramatically weakened by successive substitution of the Me3Si group at the Si-H functionality. A substantial decrease in the bond strength is also observed by replacing alkyl with methylthio groups.

In the M-R bonded intermediate an a-elimination is not possible, whereas a P-elimination produces a metal hydride. This is energetically unfavorable compared to the oxy-bonded intermediate partly because of the lower M-H bond strength compared to MO-H (see section on thermodynamics). Thus, because of kinetics (lack of decomposition pathways) and thermodynamics (energetics), the metal bound M-R intermediate is less reactive on the surface than die M-O-R intermediate. 

Lu [110] prepared the first solution phase Cl-terminated surface by immersing a Ge(l 11) sample in dilute HC1. This resulted in a well-ordered, atop adsorption, similar to the surface formed by Cl adsorption in vacuum [110]. The chloride-terminated surface product is thermodynamically favored over the hydride-terminated surface product, as the Ge-Cl bond and Ge-H bond strengths are 103 and 77kcal/mol, respectively [88]. The chloride-terminated surface demonstrates passivation in its stability against oxidation on the scale of hours in ambient air [104,111]. 

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