Hydrides, binary complex

For a long time it was thought that binary complexes of transition metals containing only o-alkyl- or aryl ligands could be thermodynamically stable only if they are coordinatively saturated (18 electron rule). A series of illuminating papers by Wilkinson 26), Lappert27) and others 28) dispelled this fallacy. It became clear that transition metal alkyls can decompose via kinetically low energy pathways such as P-hydride elimination or abstraction, but rarely via primary metal-carbon homolysis. Mechanisms and theories of decomposition have been proposed and reviewed 29). 

Some solid-state metal hydrides are commercially (and in some cases potentially) very important because they are a safe and efficient way to store highly flammable hydrogen gas (for example, in nickel-metal hydride (NiMH) batteries). However, from a structural and theoretical point of view many aspects of metal-hydrogen bonding are still not well understood, and it is hoped that the accurate analysis of H positions in the various interstitial sites of the previously described covalent, molecular metal hydride cluster complexes will serve as models for H atoms in binary or more complex solid state hydride systems. For example, we can speculate that the octahedral cavities are more spacious in which H atoms can rattle around , while tetrahedral sites have less space and may even have to experience some expansion to accommodate a H atom. 

Finally, we would like to point out several results from the work of Andrews and co-workers, involving photochemical studies of binary complexes of group 15 hydrides NH3 [80a], PHj [80b], and AsHj [80c]. 

The enzyme vanillyl-alcohol oxidase (VAO, E.C. 1.1.3.38) was examined in detail with respect to mechanism, structural properties, and biotechnological applications by van Berkel and coworkers, giving an excellent example of how detailed biochemical studies provide a basis for preparative biocatalytic applications (for recent reviews see[1, 21). The homooctamer with a monomer mass of 65 kDa was isolated and purified from Penicillium simplicissimum. The catalytic mechanism of VAO-catalyzed oxidation of para-alkyl phenols was studied in detail[3-5 After initial hydride abstraction from the Ca atom, a binary complex of the intermediate para-quinone methide and reduced FAD reacts with molecular oxygen, regenerating the... 

Kinetic studies of reversible inhibition by substrate analogs give evidence of the mode of action of the inhibitor and the types of enzyme-inhibitor complex formed, and estimates of their dissociation constants. The complexes may be isolated and sometimes crystallized. Studies of the stabilities, optical properties, and structures of ternary complexes of enzymes, coenzymes, and substrate analog in particular, as stable models of the catalytically active ternary complexes or of the transition state for hydride transfer (61,79,109,115-117), can only be touched upon here there is direct evidence with several enzymes that the binding of coenzymes is firmer in such complexes than in their binary complexes (85,93,118), which supports the indirect, kinetic evidence already mentioned for a similar stabilization in active ternary complexes. 

Indirect evidence for a relatively slow isomerization step that is rate-limiting under some conditions has also been obtained 35). The dissociation velocity constant. A -, for the compound E-NADH is increased threefold in the presence of sodium chloride, but the maximum rate of ethanol oxidation is only slightly increased thus, dissociation of NADH can no longer be the sole rate-determining step. Since the fast hydride transfer step was not affected by sodium chloride, and reasonable evidence that aldehyde dissociation is also relatively fast was obtained. Shore et al. 35) concluded that a new step in the mechanism had been revealed. This could be isomerization of either the ternary product complex or the binary complex E-NADH. Evidence of a similar slow step in the oxidation of ethanol and propanol with APAD as coenzyme 71) was referred to in Section II,E,1. 

H]pyridoxamine which was converted by aspartate aminotransferase apoca-zyme in the presence of a-oxoglutarate into pyridoxal with retention of H. A knowledge of the stereochemical course of the latter reaction which removes the C-4 Hgi atom allowed the conclusion that the [4 - H]pyridoxamine had the (R) configuration (Fig. 48). The reduction by NaB H4 must have involved [101] attack on the Re face of the w-electron system at C-4 of the species (Fig. 48, 1). The conclusion for aspartate aminotransferase has recently been confirmed by Zito and Martinez-Carrion [93] and the same approach when applied to tyrosine decarboxylase [103], also showed the hydride attack to occur on the Re face at C-4 of the coenzyme-enzyme imine bond. In both cases, therefore, the Re face at C-4 must be exposed to the solvent side in the binary complex. 

The pattern recorded above raises the question whether the change of face at C-4 to the solvent side is a mandatory requirement in the transformation of binary into ternary complexes in pyridoxal-P-dependent reactions. That this may be so was the view beginning to prevail until a timely reminder, or perhaps an undue caution, came from a more recent report by Zito and Martinez-Carrion [93]. As has already been cited, these workers repeated the earlier experiments of the Zurich School on aspartate aminotransferase confirming the Re face hydride attack at C-4 in the binary complex. However, aspartate aminotransferase carbamylated at the active site Lys-258 was used to produce the substrate-coenzyme Schiff base linkage in the ternary complex. Since the modified enzyme catalysed the half-transamination reaction ... 

Fluorometric determination of the binary complex in water Determination of selenium by hydride generation inductively coupled plasma atomic emission spectrometry... 

Hydrides are compounds that contain hydrogen (qv) in a reduced or electron-rich state. Hydrides may be either simple binary compounds or complex ones. In the former, the negative hydrogen is bonded ionicaHy or covalendy to a metal, or is present as a soHd solution in the metal lattice. In the latter, which comprise a large group of chemical compounds, complex hydridic anions such as BH, A1H, and derivatives of these, exist. 

Fig. 3.11. Numerous examples are also known in which hydrogen acts as a bridge between metallic elements in binary and more complex hydrides, and some of these will be mentioned in the following section which considers the general question of the hydrides of the elements.

Hydrido complexes are well-known bui simple binary hydrides are noi. which is iti keeping wiih ihe posinon nf ihese melals in Ihe hydrogen gap" portion of ihe periodic lable ip, 67. ... 

The reactivity of these metal hydride-metal carbonyl reactions can be correlated with the nature of the reactants in a manner consistent with the proposed mechanism nucleophilic attack by hydride on coordinated CO. Thus reactions involving the highly nucleophilic group IV hydride, Cp gZrHg, are much faster than those of group V metal hydrides. On the other hand, the relatively electrophilic neutral binary metal carbonyls all react with Cp2NbH under mild conditions (20-50° C), whereas more electron-rich complexes such as cyclopentadienylmetal carbonyls (Cp2NbH(C0), CpV(CO) ) or anionic carbonyls (V(CO)g ) show no reaction under these conditions. 

Because of their low intrinsic electronegativities, neutral late transition metals (bearing an abundance of lone pairs) can serve as good donors in nM— ctah interactions of the form (5.69a). Furthermore, transition-metal-hydride bonds (Section 4.4.1) often display sufficient covalency or polar-covalency (particularly in transition-metal cations) to serve as good acceptors in ns— ctmh interactions of the form (5.69b). In the present section we shall briefly examine the simple example of platinum dihydride (PtH2) as a water-mimic in binary H-bonded complexes with H20,... 

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