Hydrido species

R = CH3, CH3, CD3), which were characterized by single-crystal X-ray diffraction and NMR and IR spectroscopy These complexes are rare examples of first-row transition metal alkyl-hydrido species. ... 

Similar studies to the above, but more abbreviated, were also carried out with C2D4-H2 as a reactant mixture. The species formed under reaction conditions yields a single weak band at 2890 cm-1 which suggests a mono-hydrido species. There are also several very weak bands in the CD region between 2100 and 2160 cm-1 and a weak band at 1289 cm-1. 

However, when a less active olefin (e.g., diisobutylene or cyclohexene) or a liganded system (Bu3P/Co = 2/1,80 atm CO/H2, 190°C) was used, the hydrido species, e.g., HCo(CO)3PBu3, predominated throughout the reaction. The author concluded that in slower systems, initial interaction of the olefin with the hydrido species HCo(CO)3L could be the ratedetermining step. These results are complementary to those discussed (vide supra) for the rhodium carbonyl catalysis. 

The reduction to methoxy-hydrido species (7) is thought to proceed via loss of CO from the dicarbonyl complex followed by addition of hydrogen to give the dihydrido carbonyl species (9). The next step suggested (37) is hydride transfer to the carbonyl carbon to give a formyl species in which both the carbonyl carbon and the carbonyl oxygen coordinate to the metal center, i.e., 10 ... 

Addition of hydrogen to (37) could give a metal ethyl species, leading to propagation. Addition of hydrogen to (38) could yield either ethane (i.e., termination) or an ethyl-hydrido species which could then participate in a propagation sequence. There are innumerable examples of complexes... 

This finding is the consequence of the distribution of various ruthenium(II) hydrides in aqueous solutions as a function of pH [RuHCl(mtppms)3] is stable in acidic solutions, while under basic conditions the dominant species is [RuH2(mtppms)4] [10, 11]. A similar distribution of the Ru(II) hydrido-species as a function of the pH was observed with complexes of the related p-monosulfo-nated triphenylphosphine, ptpprns, too [116]. Nevertheless, the picture is even more complicated, since the unsaturated alcohol saturated aldehyde ratio depends also on the hydrogen pressure, and selective formation of the allylic alcohol product can be observed in acidic solutions (e.g., at pH 3) at elevated pressures of H2 (10-40 bar [117, 120]). (The effects of pH on the reaction rate of C = 0 hydrogenation were also studied in detail with the [IrCp (H20)3]2+ and [RuCpH(pta)2] catalyst precursors [118, 128].)... 

Gabrielsson et al. reported the aerobic oxidation of alcohols catalyzed by a cationic Cp Ir complexes bearing diamine ligands such as bipyrimidine 10 (Scheme 5.8) [35], the mechanism of which is closely related to the Oppenauer-type oxidation mentioned above. In this reaction, the deprotonation of Ir hydrido species to afford Ir species, and the reoxidation of Ir to Ir by O2, are crucial. 

From a historic point of view, metal-catalyzed or metal-promoted hydroamina-tions were first achieved with alkali metals [4]. The use of soluble transition-metal complexes as catalysts for the OHA reaction was pioneered by DuPont workers during the 1970s, the best results being obtained with Rh and Ir salts [5], Later, the finding that electron-rich Ir(I) species cleanly activated N—H bonds to form Ir-amido-hydrido species [6] opened the way to study the reactivity of these amides... 

The formation and the spectroscopic properties of the [Ru(CO) I ] species can be easily studied in different solvents by reacting RuCCO) directly with the iodide promoters (HI, Nal etc,). Thus the hydrido species HRu(00)313, which up to now was not well characterized has been obtained in concentrated solutions in different solvents at room temperature from Ru(CO) I and gaseous or aqueous HI. ( Mass spectrum M/e 567 (M+) 539 (M -CO) 511 (M+-2C0) 483 (M+-3C0)). 

The electrosynthesis of hydride complexes directly from molecular hydrogen at atmospheric pressure by reduction of Mo(II) and W(II) tertiary phosphine precursors in moderate yield has been described as also the electrosynthesis of trihydride complexes of these metals by reduction of M(IV) dihydride precursors [101,102]. Hydrogen evolution at the active site of molybdenum nitrogenases [103] is intimately linked with biological nitrogen fixation and the electrochemistry of certain well-defined mononuclear molybdenum and tungsten hydrido species has been discussed in this context [104,105]. 

Griffith and Wilkinson, in a nuclear magnetic resonance study (3), found that a hydrido complex was formed in quantitative yield on treatment of cyano-cobaltate(II) solution with sodium borohydride. A hydrido complex was also present to the extent of 3% in a solution which had not been so treated. Furthermore, saturation of the solution with hydrogen, or aging, did not increase the amount of hydrido species, and it was suggested that these latter processes involved the formation of a nonhydridic cobalt(I) species. 

The important feature is that no exceptional, contrived, or unprecedented chemistry has to be invoked to rationalize the stereoselective formation of the cw-alkene preferential rapid hydronation of the metal (or binding of an alkyne to a preformed hydrido species) naturally results in the c/s-alkene. 

In hydrido species, R3SiFeH(CC 4, the silicon atom is the most electrophilic site when R is H, but the Fe hydrogen atom takes over that role when more electronegative groups are attached to silicon (R = Cl, Ph). 

CsNb6IuH]. A completely different niobium hydrido species has been reported recently. It apparently contains an octahedral Nbe cluster with H at its center pNb-H for this compound is at 1120 cm-1 169). 

In this table we have adopted the convention (for ionic compounds) of listing the hydrido species first... 

---