Neutral hydrides

With three valence electrons, each group 13 element might be expected to form a hydride MH3. Although the existence of BH3 has been established in the gas phase, its propensity to dimerize means that B2H6 (diborane(6), 12.1) is, in practice, the simplest hydride of boron. 

We have already discussed the structure of and bonding in B2H5 (Sections 9.7 and 4.7) the reader is reminded of the presence of 3c-2e (delocalized, 3-centre 2-electron) B—H—B interactions. In worked example 12.2, the B and H NMR spectra of B2H5 are analysed. 

Consider the B NMR spectrum. There is one signal, but each B nucleus couples to two terminal H nuclei and two bridging H nuclei. The signal therefore appears as a triplet of triplets  

The exact nature of the observed spectrum depends upon the actual values of /( B- Hterminai) and /( B- Hbrfdge) 

Consider first the signal due to the terminal protons. For B, / =, meaning that there are four spin states with values + j, +5, — 5 and — There is an equal probability that each terminal H will see the B nucleus in each of the four spin states, and this gives rise to the H signal being split into four equal intensity lines a 1 1 1 1 multiplet. 

B-H-B interactions. In worked example 13.2, the B and h NMR spectra of B2H6 are analysed. 

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OsH(CO)(Hpz)2(P Pr3)2]BF4, which can be deprotonated by NaH, while treatment with HC1 gives the neutral hydride-chloro complex OsHCl(CO)(Hpz) (P Pr3)2, which can also be prepared by addition of pyrazole to OsHCl(CO) (P Pr3)2 (Scheme 37).77... 

The existence of two different rhodium species co-existing on the silica support can be used as an advantage by controlling their relative amount. Under standard hydroformylation conditions, the cationic species and the neutral hydride complex are both present in significant amounts. Hence hydroformylation and hydrogenation will both proceed under a CO/H2 atmosphere. Indeed a clean one-pot reaction of 1-octene to 1-nonanol was performed, using the supported catalyst for a hydroformylation-hydrogenation cascade reaction. 98 % of the 1-octene was converted in the... 

The cationic tantalum dihydride Cp2(CO)Ta(H)2]+ reacts at room temperature with acetone to generate the alcohol complex [Cp2(C0)Ta(H01Pr)]+, which was isolated and characterized [45]. The mechanism appears to involve protonation of the ketone by the dihydride, followed by hydride transfer from the neutral hydride. The OH of the coordinated alcohol in the cationic tantalum alcohol complex can be deprotonated to produce the tantalum alkoxide complex [Cp2(C0)Ta(01Pr)]. Attempts to make the reaction catalytic by carrying out the reaction under H2 at 60 °C were unsuccessful. The strong bond between oxygen and an early transition metal such as Ta appears to preclude catalytic reactivity in this example. 

No change in the rate or ee of the catalytic reaction was observed when the pressure of H2 was varied, indicating that H2 does not play a role in the turnover-limiting step or in the determination of enantioselectivity. When the catalytic reaction was monitored by NMR under H2 (5 bar), the neutral hydride was observed. All of these observations support the proposed mechanism shown in Scheme 7.12. This... 

This reaction probably proceeds via the neutral hydride that undergoes reductive elimination of germane. The stereochemistry observed (retention of configuration) is in agreement with a reductive elimination and with the assumption that the formation of the molybdenum-germanium bond proceeds with retention of configuration (cf. Sect. 3.1.2). 

The rhodium complex with bis(diphenylphosphino)phenoxazine was immobilized on silica using the sol-gel technique or by a direct grafting to commercially available silica [127]. Under standard hydroformylation conditions (CO/H2 atmosphere), a neutral hydridic complex (57) and cationic species (58) (Scheme 4.35) coexist on the support and act as a hydroformylation/hydrogenation sequence catalyst, giving selectively 1-nonanol from 1-octene 98% of 1-octene were converted to mainly linear nonanal which was subsequently hydrogenated to 1-nonanol. The... 

The reactions are solvent dependent. Non-polar solvents favour the formation of the neutral hydride (68), whereas in polar media, ionic species predominate. Hydroformylation activity for propylene was observed only under conditions where (68) was formed.291 The dinuclear species [Co2(CO)6(PBu"3)2] (69) also appears to be catalytically inactive.290,291 If PBu"3 is added in excess over cobalt, only complexes (68) and (69) were present. The rate of hydroformylation then became first order in hydrogen, owing to the equilibrium shown in equation (60).292... 

HCl.Aq + Zn = ZnCl2.Aq + 2H the hydrogen ion unites with the arsenic, negatively charged in the electric couple, forming electrically neutral hydride of arsenic, which escapes... 

The reaction of a carbonylmetalate with a neutral metal carbonyl has been labeled a redox condensation by Chini et al. (40, 41) and has been as widely used as a pyrolysis reaction for synthesizing mixed-metal clusters. Carbonylmetalates usually react rapidly with most neutral carbonyls, even under very mild conditions. A large number of mixed-metal hydride clusters have been formed via this type of reaction, primarily because the initial products are anionic clusters that in many cases may be protonated to yield the neutral hydride derivative. 

In many cases anionic mixed-metal clusters can be protonated to yield neutral hydride clusters, and likewise neutral hydride clusters can be deprotonated to yield anionic species. For example, [CoRu3(CO)13] is the initial product resulting from the addition of [CoICOIJ- to Ru2(CO)12. This cluster can either be isolated as its [(Ph3P)2N]+ salt or protonated with H3P04 to give CoRu3H(CO)13, Eq. (81) (147). 

Likewise, we have prepared the anionic clusters [CoFeRu2(CO)13] and [CoFe2Ru(CO)13T, but all attempts to isolate the corresponding neutral hydride clusters after protonation have failed, apparently because of their instability (147). 

In this manner are formed pentacarbonylhalogenotungsten hydrides which, to the best of our knowledge, are new, in contrast to the corresponding anions. The neutral hydride complexes are very unstable and in aqueous solution they dissociate completely into hydronium and penta-carbonylhalogenotungstate ions. The latter can be precipitated as tetra-methylammonium salts (71) ... 

The neutral hydride complex Rh(CO)(/r-DPPM)20s(H) (C0)2 reacts with propyne and allenes to form vinyl derivatives (equation 33 (R = H, Me)). That the vinyl groups are... 

Treatment of (10) with KH in THF effects stepwise deprotonation to a mono- (15) and dianion (16) excess base gives (17) (Scheme 5). When (1) is treated with KOH followed by acidification with concentrated sulfuric acid in CH2CI2, the purple neutral hydride (14) is produced. ... 

Just as further protonation of a cationic hydride can lead to Hj evolution and formation of a dication, futher protonation of a neutral hydride can lead to Hj loss and formation of a monocation. Although CF3SO3H in THF gives HMn(CO)j from [Mn(CO)5]- ... 

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