Hydrido complexes properties

Monocarbene complexes of Pt° [114] as well as dicarbene complexes of Ni° and Pd° [115] activate the C2-H bond of imidazolium salts under formation of the thermally stable bis (31) and triscarbene hydrido complexes 32 (Fig. 11). The superb donor properties of the NHC ligands in the precursor complexes most likely support the oxidative addition of the imidazolium salt. 

Further reduction to cobalt (I) further increases the electron population of the coordination center and the radical-bonding properties of cobalt are no longer favored. Instead, the EPD properties that prevail at the coordination center allow coordination by EPA units according to the second stabilizing rule the complex ion is stabilized ) as a hydrido complex ... 

The apparatus and procedures are similar to those in the preparation above and a 1L flask is used. A solution of lithium tri-ferf-butoxyhydridoaluminate 9 (28.6 g, 113 mmole) in purified THF (100 mL) is added slowly to a solution of ZrCl2(i7s-C5Hs)2 (32.9 g, 113 mmole) in THF (500 mL) with stirring. After complete addition, stirring is continued for 1 hour, after which the mono-hydrido complex is collected by anearobic filtration (Fig. 1) and washed with THF (yield 26.3 g, 90%). Anal. Calcd. for C10H ClZr ash (Zr02), 47.77% hydrolyzable H, 1.00 g-atom/mole Cl, 13.75. Found ash, 47.0% hydrolyzable H, 1.02 g-atom/mole Cl, 13.4. One-quarter mole of Li[AlH4] may be used instead of the tri-terr-butoxy hydrido complex, but the essential control of stoichiometry is more difficult (see Properties). 

The chemistry of hydrido complexes of group V metals seems to reflect the usual tendency for vanadium to behave differently from the other two elements, although generalizations are probably premature in such a new field. Certainly niobium and tantalum form numerous hydrides of similar composition and properties, appearing to have no vanadium counterparts to date, but there have been no systematic investigations involving all three elements under comparable conditions, as is clear from the following discussions. 

The properties of hydrido complexes vary widely, ranging from those which can be detected only spectroscopically to those that show remarkable general chemical stability. Stability generally tends to be at a maximum in compounds of the third transition series. Furthermore, for a given metal and type of complex, stability appears to be at its highest when... 

Table 6 Some Mononuclear Hydrido Complexes of Rhenium(III) and Their Spectroscopic Properties... 

The light orange hydrido complex melts with decomposition at 280°C. Since the hydrido ligand is coordinated to rhodium (rRh H 1902cm-1, rRh D 1465cm-1), the complex is coordinatively saturated and exhibits poor catalytic properties. 

Butadiene, Using butadiene as the model substrate, it has been found that the catalytic hydrogenation of dienes occurs in three steps (1) reversible addition of hydrido complex to butadiene to form a butenylcobalt complex (Reaction 3) (2) cleavage of the butenylcobalt complex by hydrido complex to form butenes and pentacyanocobaltate(II) (Reaction 4) and (3) hydrogen absorption by pentacyanocobaltate(II) to reform hydrido complex (Reaction 1) 20, 21), The butenylcobalt complex has the same properties as the complex obtained via reaction of y-methyl-allyl bromide with pentacyanocobaltate(II) (21, 22). 

The pale yellow [Ni(PEt3)4] is also tetrahedral but with some distortion. In sharp contrast to nickel, palladium forms no simple carbonyl, Pt(CO)4 is prepared only by matrix isolation at very low temperatures and reports of K4[M(CN)4] (M = Pd, Pt) may well refer to hydrido complexes in any event they are very unstable. The chemistry of these two metals in the zero oxidation state is in fact essentially that of their phosphine and arsine complexes and was initiated by L. Malatesta and his school in the 1950s. Compounds of the type [M(PR3)4], of which [Pt(PPh3)4] has been most thoroughly studied, are in general yellow, air-stable solids or liquids obtained by reducing complexes in H2O or H20/EtOH solutions with hydrazine or sodium borohydride. They are tetrahedral molecules whose most important property is their readiness to dissociate in solution to form... 

Methods of preparing selected hydrido complexes were given in equations 23.2 and 23.46-23.50. Selected properties of the mononuclear complexes HMn(CO)5, H2pe(CO)4 and HCo(CO)4 are given in Table 23.6. Metal hydrides play an important role in organometallic chemistry, and scheme 23.57 illustrates some ligand transformations involving M—H bonds HCo(CO)4 is an industrial catalyst (see Section 26.4). 

The hydrido complex [(l,5-C8Hi2)RhH]4 can be used as a source of small rhodium crystallites. Aromatic hydrocarbon solutions of this complex are unstable when exposed to dihydrogen, allowing the formation of Z nm size, crystalline but agglomerated Rh nanoparticles, that show catalytic properties in aromatic hydrocarbures hydrogenation. ... 

Anal. Calcd. for C,oHaClZr. ash (ZrOj), 47.77% hydrolyzable H, 1.00 g-atom mol" Cl, 13.75. Foimd ash, 47.0% hydrolyzable H, 1.02 g-atoms mol Cl, 13.4. One-quarter mol of Li[AlH4] may be used instead of the tri-tert-butoxy hydrido complex, but the essential control of stoichiometry is more difficult (see Properties section). 

Geometry and Properties of Transition Metal Hydrido Complexes... 

Strong acidic metal hydrido complexes such as HCofCO) or complexes with Lewis acid properties, such as Rh2Cl2(CO)4, [Ru(MeCN)3(triphos)](CF3S03)2, [Pt(H20)2(dppe)](CF3S03)2, [Pd(H20)2(dppe)](CF3S03)2, or [Ir(MeCN)3(tri-phos)] (CF3S03)3, are able to act in alcohols as acetahzation catalysts, which means they can mediate the transformation of the newly formed aldehydes into acetals (see Section 5.3). 

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