Dihydrogen with metals

INTERACTION OF DIHYDROGEN WITH METAL SURFACES, METAL OXIDES AND HYDRIDES, AND NONTRANSITION METAL SYSTEMS... 

The standard reduction potential for Be2+ is the least negative of the elements in the group and by the same token beryllium is the least electropositive and has the greatest tendency to form covalent bonds. The bulk metal is relatively inert at room temperature and is not attacked by air or water at high temperatures. Beryllium powder is somewhat more reactive. The metal is passivated by cold concentrated nitric acid but dissolves in both dilute acid and alkaline solutions with the evolution of dihydrogen. The metal reacts with halogens at 600°C to form the corresponding dihalides. 

The ability of transition-metal complexes to activate substrates such as alkenes and dihydrogen with respect to low-barrier bond rearrangements underlies a large number of important catalytic transformations, such as hydrogenation and hydroformy-lation of alkenes. However, activation alone is insufficient if it is indiscriminate. In this section we examine a particularly important class of alkene-polymerization catalysts that exhibit exquisite control of reaction stereoselectivity and regioselec-tivity as well as extraordinary catalytic power, the foundation for modern industries based on inexpensive tailored polymers. 

As shown in Figure 53, the interaction of dihydrogen with the metal centre of transition metal complexes can be described as a two-electron... 

V(OH)2 is a strong reducing agent and freshly prepared V(OH)2 reacts with water with dihydrogen evolution. In acidic solution, reduction of water may be induced by UV radiation.138 From solutions containing complexes with catechol there is evolution of dihydrogen with simultaneous oxidation of the metal to vanadium(III). The reaction is first order in vanadium(II) and autocatalytic (Scheme 7).145... 

Greg Kubas of Los Alamos National Laboratory has determined how hydrogen interacts with metals. The important part of his work is that hydrogen, a substrate that is redox inactive substrate and not Brpnsted acidic, transforms upon complexation whereupon the coordinated H2 becomes acidic. The deprotonation of a metal dihydrogen complex generates oxidizable species and in this way, H2 is connected to electrons and heterolytic activation. Rauchfuss explained that Kubas discovery has helped guide his team s effort to connect H2 binding to this redox-active iron metal. 

Carbon monoxide has a high affinity for transition metals, forming the metal carbonyls (see 14.6.2.). Despite this, CO reacts slowly or not at all with metals. Some finely divided metals (Fe, Co) are converted slowly to the corresponding carbonyls under drastic conditions of T and P. Active Ni, as it is obtained by reducing nickel oxide with dihydrogen at 400°C, is, on the other hand, easily carbonylated to Ni(CO)4 even at temperatures as low as 30°C ... 

Four-coordinate Compounds. - The geometry of the phosphate group (and its interactions with metal cations) has been studied in the crystal environment by analysing XRD data from 178 crystal structures, and in vacuo by ab initio calculations of dihydrogen- and dimethyl phosphates and a diphosphate model... 

One of the key advantages of supercritical solvents over conventional liquids is their unique combination of gas-like and liquid-like properties. Thus, reactant gases such as H2 or N2 can, under the correct conditions of temperature and pressure, be totally miscible with SCF solutions. At Nottingham, this unique property has been exploited to synthesize a range of novel organometallic complexes with metals bonded directly to dinitrogen [17] and dihydrogen [7,18], as shown in eq (3.1.1). These experiments are outlined in more detail in chapter 4.2. 

About 400 compounds containing the T12-H2 ligand have been made that are stable. Almost all of them are octahedral with the metal in the d electron configuration, from Cr(0) to W(0) across the periodic table to Rh(lll) and Ir(III). There are a few dihydrogen complexes known with metals in other oxidation states. These are relatively rare. There are seven coordinate rhenium compounds like [Re(H2)(H)2(CO)(PMe2Ph)3]+ [8] and very unstable d systems like [Pt(ll2-H2)(PR3)2H]+ [9]. Neutral and cationic complexes are known but not anionic ones. Presumably anionic dihydrogen complexes are unstable because the metal s dTC electron richness promotes the oxidative cleavage of the H-H bond by d7t(M) —> o (H2) donation. 

Finally, it is worth noting that there is a hydrogenase which apparently does not contain any metal [6]. N5,N10-methylenetetrahydromethanopterin dehydrogenase, isolated from methanogenic bacteria is capable of producing and consuming dihydrogen with coenz)nne F-420 (deazaflavin) as acceptor. 

Weak adducts of alkanes and metal derivatives (the alkane molecules play the role of token ligands in these complexes) have been detected and even isolated using a number of methods [14], These complexes are unstable at room temperature. Matrix isolation is one of the best established methods for the stabilization and characterization of intermediates. Complexes of alkanes with metal atoms and ions have been detected in the gas phase. All these adducts belong to the larger class of cr-complexes, which has been defined as complexes where the donor is a o-bond [14c], Dihydrogen and silane complexes are also from this class. 

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