Clusters hydride

Since metal clusters containing hydrogen directly bonded to the metallic nucleus are often involved in the synthesis, reactivity, and catalytic properties of cluster species, they have been frequently mentioned in the chemistry discussed in Chapter 2. Nonetheless, some features related to the structure of and bonding in these compounds will be discussed separately in this Section. 

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The nomenclature of boron hydride derivatives has been somewhat confusing and many inconsistencies exist in the Hterature. The stmctures of some reported boron hydride clusters are so compHcated that only a stmctural drawing or graph, often accompanied by explanatory text, is used to describe them. Traditional nomenclature systems often can be used to describe compounds unambiguously, but the resulting descriptions may be so long and unwieldy that they are of Htde use. The lUPAC (7) and the Chemical Abstract Service (8) have made recommendations, and nomenclature methods have now been developed that can adequately handle nearly all clusters compounds however, these methods have yet to be widely adopted. Eor the most part, nomenclature used in the original Hterature is retained herein. 

Sodium borobydride reacts with Lewis acids in nonprotic solvents to yield diborane [19287-45-7] 2 6 which can then be used to generate other useful organoboranes such as amine boranes, alkyl boranes, and boron hydride clusters. 

Regioselective coniugale reduction and reductive silylation of a,p-unsaturated ketones, esters, and aldehydes using a stable copper (I) hydride cluster (Ph3P)CuH 6... 

N. N. Greenwood, Boron Hydride Clusters, in H. W. Roesky (ed.) Rings, Clusters and Polymers of Main Group and Transition Elements, Elsevier, Amsterdam, 1989, pp. 49-105. 

Kinetic studies of the hydride cluster [W3S4H3(dmpe)3] with acids in a non-coordinating solvent, i.e., dichloromethane, under the pseudo-first-order condition of acid excess, show a completely different mechanism with three kineti-cally distinguishable steps associated to the successive formal substitution of the coordinated hydrides by the anion of the acid, i.e., Ch in HCl [37]. The first two kinetic steps show a second-order dependence with the acid concentration. 

The stable hydride cluster Pd4(dppm)4(H)3]+ [dppm = bis(diphenylphosphino)methane] can be electrochemically generated from Pd2(dppm)2Cl2] at —1.35 V vs. SCE in DMF containing HC02H (Equations (21) and (22)). Subsequent addition of proton under the same reducing conditions induces dihydrogen evolution.43... 

The modified BINAP catalyst 5 has been used for the hydrogenation of a number of analogues of substrate 1 (substrates 32-35, Fig. 30.8 Table 30.6), though again, enantioselectivities were modest [4]. Substrate 31 has also been hydrogenated with a ruthenium-BINAP-hydride cluster with low selectivity (11% ee) [27]. 

Conjugate reduction.1 This stable copper(I) hydride cluster can effect conjugate hydride addition to a,p-unsaturated carbonyl compounds, with apparent utilization of all six hydride equivalents per cluster. No 1,2-reduction of carbonyl groups or reduction of isolated double bonds is observed. Undesirable side reactions such as aldol condensation can be suppressed by addition of water. Reactions in the presence of chlorotrimethylsilane result in silyl enol ethers. The reduction is stereoselective, resulting in hydride delivery to the less-hindered face of the substrate. 

Table 10. High field H NMR data for tetranuclear hydride clusters. Figures in parentheses refer to relative intensities and d = doublet, t = triplet, qu = quintet... 

Intramolecular Dihydrogen Bonds in Metal Hydride Clusters... 

A very interesting and complex protonation mechanism has been snggested for the hydride cluster [W3S4H3(dmpe)3]PF6 in CH2CI2 solutions. In the presence of an excess of HCl, a careful kinetic study of the process in eq. (10.4) by the stopped-flow technique [9] has revealed three kinetically distinguishable steps very fast, fast, and slow, with rate constants A 1, ki, and k3. The kinetic order in the initial hydride cluster in the slow step has been measured as 1. At the same time, rate constants k and A 2 have corresponded to a second-order dependence on acid concentration, while the third step has shown a zero kinetic order on HCl. The rate constants have been determined as A i =2.41 x 10 M-2/s, k2 = 1.03 X 10 M /s, A 3 = 4 X 10 s . Note that the protonation process becomes simple at lower concentrations of HCl. Under these conditions it shows a single step with a first kinetic order on the acid. 

Carbon tetrabromide, irradiation of, 5 152 Carbon tetrachloride, ionization, 9 230-231 Carbonyl anion and hydride clusters, 13 478-480, 482, 483... 

Relationships between Carbonyl Hydride Clusters and Interstitial Hydrides... 

In the penta-, hexa-, and heptanuclear carbonyl hydride clusters, terminal hydrides are observed only in the less crowded species, and again no evidence for the presence of interstitial hydride is found. 

Early Studies on Rhenium-Carbonyl-Hydride Clusters and Related Species... 

Osmium-Carbonyl-Hydride Clusters and Related Ruthenium Complexes. Our investigation of these species began with a study of the species (/i2-H)(H)Os3(CO)n, prepared from Os3(CO)i2 via the unsaturated species (M2-H)20s3(CO)io (33) (see Reaction 1). 

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