Hydride ligands acidities

A ruthenium porphyrin hydride complex was lirst prepared by protonation of the dianion, [Ru(TTP) in THF using benzoic acid or water as the proton source. The diamagnetic complex, formulated as the anionic Ru(If) hydride Ru(TTP)(H )(THF)l , showed by H NMR spectroscopy that the two faces of the porphyrin were not equivalent, and the hydride resonance appeared dramatically shifted upheld to —57.04 ppm. The hydride ligand in the osmium analogue resonates at —66.06 ppm. Reaction of [Ru(TTP)(H)(THF)j with excess benzoic-acid led to loss of the hydride ligand and formation of Ru(TTP)(THF)2. 

Replacement of a hydride ligand by a methyl substituent decreases both the thermodynamic and the kinetic acidity of the remaining hydrogen, while its replacement by an additional Os(CO) H unit increases the thermodynamic acidity but decreases the rate of deprotonation. The same additional delocalization that decreases the pK of 0so(C0)oHo relative to that... 

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 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 ... 

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. 

Scheme 3. The synthesis of 8 and 9. (i) p-loluenesulfonic acid, ClljCh (ii) CHjCyi IiO (iii) H SCVMeC. N, -30°C. The location of the hydride ligands in 8 are not known with certainty.

In coordinatively saturated metal hydrides, such as the HM(CO)s (M => Cr, Mo, W) derivatives, formation of the four-centered transition state for C02 insertion (Scheme 1) may proceed with or without CO loss and concomitant coordination ofC02 at the metal center. That is, C02 insertion may occur by means of dissociative (D) or dissociative interchange (Id) processes, or an associative interchange (Ia) process (47, 48). In either instance an acid-base interaction between the anionic hydride ligand and the electrophilic carbon center of carbon dioxide as represented in 6 may occur prior to formation of the four-centered transition state depicted in Scheme 1. An interaction of this type has been observed for these HM(CO)j derivatives with Lewis acids such as BH3 (49). 

Another recent example of the question of the formation of intermediate metal-C02 complexes in these reactions was the theoretical study by Ohnishi et al. [84] of the hydrogenation of C02 to formic acid by Ru catalysts. In the presence of water, there was no direct metal coordination of C02, but formation of adducts in which the C and O atoms of C02 interacted with the H (hydride) ligand and the H atom of H20 rfs-Ru(H)2(PMe3)3(H20)(C02). In the absence of water molecules,... 

Reactions. Cp2Nb(H)(CO) reacts with Lewis acids to produce compounds, of varying stability, in which the acid coordinates to the hydride ligand. For example,... 

Pioneering studies with semi-empirical methods were carried out shortly after the discovery of the FeMoco structure via X-ray diffraction experiments (21,22). These extended-Hiickel calculations have been carried out for structurally fixed models of FeMoco (23,24) and of FeMoco, FeVco, and FeFeco (25), in which the three amino acid ligands have been replaced by hydride ligands. The main results of these studies are deduced based on the assumption that the character of the frontier orbitals, i.e., their composition in terms of atomic orbitals,... 

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