Transition metal complexes hydration

The hydration of alkynes represents a prime example in which simple coordinative activation by transition metal complexation greatly facilitates an otherwise very slow chemical process (Equation (107)). This reaction has been a long-studied problem, but only recently have alternatives to the classical use of catalysts such as Hg(n) salts been sought. These new catalyst systems typically display much enhanced reactivity, and some can mediate an anti-Markovnikov hydration through a novel mechanism (Table 1). 

With an electrophilic transition metal complex, it is believed that the hydration of an alkyne occurs through a trans-addition of water to an 72-alkyne metal complex (Scheme 15, path A),380 although the m-pathway via hydroxymetallation has also been proposed (path B).381,382 However, distinguishing between the two pathways is difficult due to the rapid keto-enol tautomerization that renders isolation of the initial water adduct challenging. 

Cations come in many shapes and sizes. The simplest is the lone proton which may jump from base to base along a small channel. Then there are inorganic ions with no directional preferences for bonding, such as the alkali or alkaline metals, and NH4+ which is tetrahedral but appears spherical when hydrated. At the other end of the spectrum of structural complexity we have organic cations and hydrated transition metal complexes with non-uniform charge densities. 

Cytochrome c, a small heme protein (mol wt 12,400) is an important member of the mitochondrial respiratory chain. In this chain it assists in the transport of electrons from organic substrates to oxygen. In the course of this electron transport the iron atom of the cytochrome is alternately oxidized and reduced. Oxidation-reduction reactions are thus intimately related to the function of cytochrome c, and its electron transfer reactions have therefore been extensively studied. The reagents used to probe its redox activity range from hydrated electrons (I, 2, 3) and hydrogen atoms (4) to the complicated oxidase (5, 6, 7, 8) and reductase (9, 10, 11) systems. This chapter is concerned with the reactions of cytochrome c with transition metal complexes and metalloproteins and with the electron transfer mechanisms implicated by these studies. 

Beck and coworkers211 have reviewed transition metal complexes of the fulminate ion, CNO, which bonds to the metal via the carbon atom. Fulminate complexes are, in general, similar to those with CN ligands. An extensive series of stable, non-explosive fulminates has been examined, including [Ir(CNO)6](AsPh4)3. [Ir(CNO)6]3- was prepared from hydrated iridium(III) chloride and Hg(CNO)2, and characterized by IR spectroscopy.21"1... 

V). c. In aqueous solution, Co3 +, forms the hydrated transition metal complex, Co(H20)63 +. In both complexes, Co(H20)g3+ and Co(en)33+, cobalt exists as Co3+, which has 6 d electrons. Assuming a strong-field case, the d-orbital splitting diagram for each is... 

A simple ionic bonding model accounts for many properties of transition metal complexes, including variations in the hydration and lattice enthalpies and the ionic radii of the metal ions. The observation of high- and low-spin states for complexes of some metal ions can also be explained. 

The contribution of pulse radiolysis to general chemistry is very significant, and this is exemplified by the following studies of transition metal complexes. The reduction of tris(2,2 -bipyridine)ruthenium(III) ion by the hydrated electron was the first example of this type of reaction to show clearly the formation of a product in an electronically excited state [80] ... 

The structure of bis(salicylaldoxime)beryllium has been proposed as being trans octahedral by comparison of the space group and unit cell volume with those of related transition metal complexes it is presumably a (hydrate if it is indeed octahedral in geometry. Stability constants have been reported for a range of beryllium j3-ketoamines derived from both salicylaldehyde and acetylacetone precursors. They show strong complexes which are stable to hydrolysis under the conditions used. ... 

When light hits the complex, some of the energy is absorbed by the transition metal complex, and this is then observed by someone looking at the complex. For example, take [Ti(H20)] % a hydrated Ti ion, this has a reddish-violet color. This occurs because the yellow and green part of the light is absorbed by the complex, the energy in the light is used up to excite the electron. And so the transmitted (observed) color is complementary to the absorbed color. 

---