Carbonyls, metal containing hydrides

There are three important routes to the formation of the mercury-transition metal bond (a) displacement of halogen or pseudohalogen from mercury(II) salts with carbonyl metallate anions (b) reaction of a halo-phenylmercury compound with a transition metal hydride and (c) oxidative addition of a mercury halide to neutral zero valent metals.1 We report here the syntheses of three compounds containing three-centre, two-electron, mercury-ruthenium bonds utilizing trinuclear cluster anions and mercury(II) halides.2-4... 

A variety of chiral auxiliaries have been used to direct the course of carbonyl reductions by hydride reducing agents. The great majority of them contain heteroatoms capable of coordinating to metal ions, and appear to owe their effectiveness either to their ability to participate in chelated intermediates or to the ability of Lhe heteroatom to bind to a reagent and control its direction of approach. 

The NVE is often equal to 18 for transition-metal organometallics and for many inorganic complexes. This 18-electron rule should be better viewed as a strong tendency than a rule, but it is followed by a majority of complexes (despite many exceptions, vide infra). The 18-electron electronic structure often brings a good stability for complexes. For instance, this is the case for the metal carbonyl complexes, possibly the largest family. Transition-metal sandwich complexes are more stable in the 18-electron count than in others. The 18-electron electronic structure is also found most of the time in complexes containing a mixture of carbonyls, hydrocarbons, carbenes, hydrides, etc. - ... 

The metals bonded to two cyclopentadienyls (Cp) in a pentahapto mode form the metallocenes (see Chap. 11 also describing the Cp-M orbitals). In many other families of compounds, the metal is bonded to only one Cp or derivative. Common examples of this category include ternary families containing, besides the Cp-type ligand, carbonyls, phosphines, nitrosyls, hydrides, 0x0, imido, methyls, etc. - ... 

This report deals with those publications from 2000 that described advances in metal carbonyl chemistry. This includes those studies that explore the chemistry of the simple metal carbonyls themselves [such as Mo(CO)6 and Fe(CO)s], metal carbonyl halides [e.gf. Mn(CO)sBr], pseudohalides le.g. Mn(CO)5(CN)] and hydrides [e.g. Re(CO)5H], This section is not restricted to mononuclear species. Indeed, carbonyl complexes containing more than one metal atom have formed a large part of this report for many years and the percentage of polynuclear carbonyl complexes is, if anything, increasing. 

Greater selectivity in purification can often be achieved by making use of differences in chemical properties between the substance to be purified and the contaminants. Unwanted metal ions may be removed by precipitation in the presence of a collector (see p. 54). Sodium borohydride and other metal hydrides transform organic peroxides and carbonyl-containing impurities such as aldehydes and ketones in alcohols and ethers. Many classes of organic chemicals can be purified by conversion into suitable derivatives, followed by regeneration. This chapter describes relevant procedures. 

Because they possess an odd number of valence electrons the elements of this group can only satisfy the 18-electron rule in their carbonyls if M-M bonds are present. In accord with this, mononuclear carbonyls are not formed. Instead [M2(CO)s], [M4(CO)i2] and [M6(CO)i6] are the principal binary carbonyls of these elements. But reduction of [Co2(CO)g] with, for instance, sodium amalgam in benzene yields the monomeric and tetrahedral, 18-electron ion, [Co(CO)4] , acidification of which gives the pale yellow hydride, [HCo(CO)4]. Reductions employing Na metal in liquid NH3 yield the super-reduced [M(CO)3] (M = Co, Rh, Ir) containing these elements in their lowest formal oxidation state. 

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 421). Electrocyclic and sigmatropic rearrangements are treated at 18-27-18-35. Double-bond migrations have also been accomplished photochemically, and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts. In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the nwtal hydride addition-elimination mechanism ... 

Hi ly dispersed supported bimetallic catalysts with bimetallic contributions have been prepared from molecular cluster precursors containing preformed bimetallic bond [1-2]. For examples, extremely high dispersion Pt-Ru/y-AUOa could be prepared successfully by adsorption of Pt2Ru4(CO)ison alumina [2]. By similar method, Pt-Ru cluster with carbonyl and hydride ligands, Pt3Ru6(CO)2i(p3-H)(p-H)3 (A) was used in this work to adsorb on MgO support. The ligands were expectedly removable from the metal framework at mild conditions without breaking the cluster metal core. 

Rhin(bpy)3]3+ and its derivatives are able to reduce selectively NAD+ to 1,4-NADH in aqueous buffer.48-50 It is likely that a rhodium-hydride intermediate, e.g., [Rhni(bpy)2(H20)(H)]2+, acts as a hydride transfer agent in this catalytic process. This system has been coupled internally to the enzymatic reduction of carbonyl compounds using an alcohol dehydrogenase (HLADH) as an NADH-dependent enzyme (Scheme 4). The [Rhin(bpy)3]3+ derivative containing 2,2 -bipyridine-5-sulfonic acid as ligand gave the best results in terms of turnover number (46 turnovers for the metal catalyst, 101 for the cofactor), but was handicapped by slow reaction kinetics, with a maximum of five turnovers per day.50... 

Some reactions of carbonyl hydrides will be illustrated in Chapter 22. Such species are involved in catalytic processes in which metal carbonyls function as hydrogenation catalysts. Generally carbonyl hydrides are obtained by acidifying solutions containing the corresponding carbonylate anion or by the reactions of metal carbonyls with hydrogen. The following reactions illustrate these processes ... 

Imine formation is an important reaction. It generates a C-N bond, and it is probably the most common way of forming heterocyclic rings containing nitrogen (see Section 11.10). Thns, cycliza-tion of 5-aminopentanal to A -piperideine is merely intramolecular imine formation. A further property of imines that is shared with carbonyl groups is their susceptibility to reduction via complex metal hydrides (see Section 7.5). This allows imines to be... 

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