Acids transition metal hydrides

Thermodynamic data on the acidity of organometallic hydrides should help identify situations where apparent reactions of acidic transition-metal hydrides actually result from their conjugate bases. A case in which both species can react but give different products (as was pointed out by Prof. Espenson three years ago (18)) is the addition of hydridocobaloximes, HCo(dmgH)2B, to olefins with electron-withdrawing substituents... 

Acidic transition-metal hydrides have been used to prepare metal-silicon bonded compounds by an interesting condensation reaction that proceeds with amine elimination (equation 30)66. An attempt to carry out a similar reaction between Ph3SiH and the zirconium amide Cp2Zr(NMe2)2 failed67. 

HCo(CO), an exceptionaUy acidic transition metal "hydride", is formed by tfie reaction of Hj and COjfCOlg. HCo(CO)g is a trigonal bipyramidal d , 18-electron complex in which the hydride occupies an apical position. High temperature is required for industrially useful rates of hydroformylation catalyzed by HCo(CO)g. Moreover, high pressure of CO is required to prevent formation of higher cobalt clusters and of metallic cobalt. The rate of hydroformylation catalyzed by cobalt carbonyl depends on [H ] and Thus, increasing the pressure of a 1 1 mixture of CO Hj has little effect on tire rate but prevents catalyst decomposition. 

The Bronsted acidity of transition metal hydrides. R. G. Pearson and P. C. Ford. Comments Inorg. Chem., 1982, 1, 279-291 (33). 

It is known that transition metal hydrides exhibit acidic properties and it has been reported that in some cases complexes containing a group IV metal ligand react with weak bases ... 

One must always keep in mind that in aqueous solutions the transition metal hydride catalysts may participate in further (or side) reactions in addition to being involved in the main catalytic cycle. H and P NMR studies established that in acidic solutions [RhCl(TPPMS)3] gave cis-fac-and ci5-7 er-[RhClH2(TPPMS)3] [86,88], while in neutral and basic solutions these were transformed to [RhHX(TPPMS)3] (X = H2O or Cl ) [86]. Simultaneous pH-potentiometiic titrations revealed, that deprotonation of the dihydride becomes significant only above pH 7, so this reaction of the catalyst plays no important role in the pH effects depicted on Figs. 3.2.a and 3.2.b. 

One of the important properties of dihydrogen ligands, particularly in charged transition metal complexes, is their ability to nndergo heterolytic cleavage [9]. In addition, protonation of transition metal hydrides with acids is a common method for preparation of transition metal dihydrogen complexes ... 

Most transition metal hydrides do not exhibit such behavior since they tend to be covalent or acidic in nature, but several examples of hydridic behavior have been reported ... 

Hydrides of Pt(II) are the most numerous of any transition metal hydride group. In addition to the presence of the hydride ligand, the complexes invariably have a coordinated phosphine, and synthetic routes to these compounds using both hydridic and protonic reagents have been reported (I). The pure complexes are usually both air stable and kinetically inert. The purpose of this chapter is to show the diversity of hydrides that can be obtained from protonation reactions on zero-valent and di-valent triphenylphosphine platinum compounds, and to rationalize the type and nature of the product formed from the character of the acid HX. 

Trans effect, 543-545 Transferrins, 937 Transition metal hydrides, acidities of. 643... 

Silyl-substituted transition-metal hydrides show remarkably high acidities, which decrease in the sequence... 

Table 1. Acidity of Neutral and Anionic Transition-Metal Hydrides...

Many transition-metal hydrides exhibit significant equilibrium acidities [12]. This applies in particular to those that have electron-withdrawing ancillary ligands and/ or a positive charge. This section presents a thermochemical cycle that uses these acidity (pAa) data, when available, as an anchoring point to derive absolute BDE data. In principle the method is applicable to the investigation of BDEs of any M-X bond. In practice, applications have been limited to M-H because data on the prerequisite equilibrium dissociation of M-X to M and X+ in solution have been limited to metal hydrides. 

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