Adducts, metal-water formation

Our studies have shown that almost all metal atoms will form an adduct with water and in many instances undergo further reaction. Theory and experiment (1 -7 ) support the concept of water acting as a Lewis base which donates electron density to the metal. Theory has also shown for Be and Mg that donation occurs extensively from the 3ai (a lone pair) orbital of water to the metal. This is of interest since this orbital is largely responsible for the nonlinearity of the water molecule. It is known, for instance, that ionization from this orbital causes water to become linear and results in a decrease in its bending frequency by approximately 700 cm . This is consistent with our finding that only the V2 bending mode of water decreases measurably upon adduct formation. The observed Av2 changes for metal-water adducts are compared and discussed below. 

FIGURE 1.2. Formation of nanoparticles of metal oxide by reverse micelle method. A solution of inverse micelles is first formed by adding a long-chain alkylamine to a toluene solution. A small amount of water is trapped in the reverse micelle core. Mixing the reverse micelle solution with an aluminum alkoxy amine adduct results in hydrolysis of the aluminum alkoxide adduct and formation of nano-sized particles of aluminum oxyhydroxide after drying. These particles are shown in the SEM picture above. 

In the calculations of the energy of hydration of metal complexes in the inner coordination sphere, one must consider hydrogen bond formation between the first-shell water molecules and those in bulk water, which leads to chains of hydrogen-bonded water molecules. Such hydrogen-bonded chains of ethanol molecules attached to the central metal ion have been found as a result of DFT B3LYP calculations on ethanol adducts to nickel acetylacetonate, where the calculated energy of hydrogen bonds correlated well with experimental data [90]. 

Another recent example of the question of the formation of intermediate metal-C02 complexes in these reactions was the theoretical study by Ohnishi et al. [84] of the hydrogenation of C02 to formic acid by Ru catalysts. In the presence of water, there was no direct metal coordination of C02, but formation of adducts in which the C and O atoms of C02 interacted with the H (hydride) ligand and the H atom of H20 rfs-Ru(H)2(PMe3)3(H20)(C02). In the absence of water molecules,... 

The formation of vinylcarbamates is restricted to terminal alkynes, which is in line with the formation of a metal vinylidene intermediate, and also to secondary amines. Indeed, a catalytic reaction also takes place under similar conditions with primary aliphatic amines but it leads to the formation of symmetrical ureas (Scheme 3) [10]. The catalytic system generated in this case is also thought to proceed via a ruthenium vinylidene active species and is very efficient for the formal elimination of water by formation of an organic adduct. The proposed general catalytic cycle, which applies for the formation of vinylcarbamates and ureas, is shown in Scheme 4 [11]. 

The effectiveness of various substituted BINOL ligands 12-16 in the Zr(IV)-or Ti(IV)-catalyzed enantioselective addition of allyltributyltin to aldehydes was also investigated by Spada and Umani-Ronchi [21], The number of noteworthy examples of asymmetric allylation of carbonyl compounds utilizing optically active catalysts of late transition metal complexes has increased since 1999. Chiral bis(oxazolinyl)phenyl rhodium(III) complex 17, developed by Mo-toyama and Nishiyama, is an air-stable and water-tolerant asymmetric Lewis acid catalyst [23,24]. Condensation of allylic stannanes with aldehydes under the influence of this catalyst results in formation of nonracemic allylated adducts with up to 80% ee (Scheme 3). In the case of the 2-butenyl addition reac-... 

SI, Ge, Sn and Pb metal atoms initially reacted with water on cocondensation to form the metal atom-water molecule adduct as has been generally observed for metal atom-water reactions ( ). The shift in the water bending mode frequency on adduct formation decreases with increasing atomic number as illustrated in Table I. Using the results obtained from theoretical studies ( 3, 5), it is believed that the water bending mode shift might serve as a relative measure of the extent of interaction between the water molecule and the metal atom, for metals within a particular group. 

The above discussion has focused on the structures of the insertion products. The remainder of this section will briefly discuss the equally Important problem of the dynamics of metal atom-water interactions. Of special Interest is the possible role of the adducts in the route to the insertion products. The matrix isolation experiments of Hauge and co-workers ( 1- ) suggest that for most metal atoms the first step of the reaction Involves formation of the adduct. Upon irradiation the adduct can Interconvert to the insertion product, which, in turn, can give rise to still other products. Two exceptions to this are Sc and A1 which in the matrix isolation experiments react spontaneously to give the hydroxyhydrides. Upon irradiation these decompose to yield metal oxides and hydroxides. 

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