Oxidation State Chemistry

One of the major growth areas in organolanthanide chemistry involves the chemistry of the metals in lower oxidation states. Prior to 1978, with the exception of a few, brief synthetic reports on divalent species, organolanthanide chemistry involved almost exclusively the -1-3 oxidation state (5). In the late 1970s, major research efforts in low-valent lanthanide chemistry were initiated 14, 60) and they served to stimulate the development of the entire organolanthanide area. 

There are two low-valent oxidation states available to the lanthanides under normal conditions the +2 oxidation state and the formally zero oxidation state found in the elemental metals. The zero oxidation state is available to all the lanthanides, but only three members of the series have +2 oxidation states accessible under common organometallic reaction conditions Eu (4/ ), Yb (4/ ), and Sm (4/ ). The Ln VLn reduction potentials [vs. normal hydrogen electrode (NHE)] (12), - 0.34 V for Eu, - 1.04 V for Yb and - 1.50 V for Sm, indicate that Eu is the most stable and Sm the most reactive of these divalent ions. Sm is also the most reactive based on radial size considerations, since it is the largest and most difficult to stabilize by steric saturation. 

Some of the earliest studies of organolanthanide chemistry described reactions of the elemental metals with alkyl and aryl iodide reagents (RI) (67). Analysis of the soluble products obtained for Ln = Eu, Yb, Sm indicated a formula of primarily RLnl although it was acknowledged that this could represent a number of different species in equilibrium. The amount of contamination of the divalent product with trivalent species was observed to follow the order of stability of the divalent states the Eu system was the cleanest, while the Sm system had only 50% of the metal in the divalent state. These species reacted like Grignard reagents. The 

A second, early approach to elemental lanthanide chemistry involved transmetallation reactions of Yb, Eu, and Sm with R2Hg reagents [Eq. (23)] (63, 64). 

In addition to divalent perfluoroaryl products, divalent alkynide complexes, Yb(C=CR)2, were prepared by this route (65, 66). 

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Higher oxidation state chemistry of iron, cobalt and nickel. W. Levason and C. A. McAuliffe, Coord. Chem. Rev., 1974, 12, 151-184 (398). 

Amongst the consequences to be expected from the change from Werner-type behaviour to carbonyl, low oxidation state chemistry is a breakdown in the efficacy... 

We attempt in Table 4 to give an overview of the range of oxidation states covered by the organometallic chemistry of manganese, and thus the bulk of the lower oxidation state chemistry... 

Beyond the 3+ oxidation state, rhodium forms a limited number of complexes in the 4+, 5+ and 6+ oxidation states. A recent review1221 gives an excellent summary of the chemistry of the higher oxidation state chemistry of Rh (as well as Ru, Os, Ir, Pd and Pt). For the 4+ oxidation state, the hexafluoro, hexachloro and trioxo dianions are well characterized. The known neutral species include RhF4 and some oxides. There are also scattered reports of Rhlv complexes containing substituted biguanides and Schiff base chelates. 

The coordination chemistry of boron was reviewed some time ago and the structure and properties of compounds of the general formula BX3 L, where X and L can be one of a wide variety of substituents and electron pair donors, respectively (15). Indeed, the reactions of tricoordinate boron compounds in general are thought to proceed via addition of the reaction partner in a Lewis acid-base reaction to yield a tetracoord-inate intermediate that then undergoes further reaction. Stable tetra-coordinate boron compounds are subject to ligand displacement reactions for which a variety of mechanisms obtain (16). The coordination chemistry of transition metals is vast and includes not only structimal facts (17) but considerable information on the mechanistic behavior of these species as well (18). In our brief comparison we will restrict ourselves to low oxidation state chemistry and group 16 metals (19). 

Joseph Chatt s interests in organometallic chemistry were wide-ranging, from bonding theories to nitrogen fixation. While these areas may not seem of immediate relevance to either carbonyl cluster chemistry or to electrospray mass spectrometry (both of which play a major role in the work described herein), the chemistry that is discussed broadly overlaps with Chatt s contributions in metal hydride, metal phosphine and low-oxidation-state chemistry. 

Both our photochemical and radiation studies have focused on the chemistry of very reactive species in aqueous solution. Indeed, it is because the photochemical work involved aqueous media that radiation chemistry techniques could be so useful to us. Our pulse radiolysis work has led to a number of highly unusual mechanistic conclusions. In the area of low-oxidation-state chemistry, several of the systems violate standard organometaUic dogma. We investigated the rate of hydride formation in another cobalt(I) system, that derived from the high-spin d polypyridyl-cobalt(I) complexes (28). Remarkably, electron transfer was found to be the rate-determining step for formation of the hydride complex, and contributions from Bronsted acid pathways contribute neghgibly to the rate. Rather, the hydride formation appears to involve H-atom transfer from the protonated bpy radical. The H-atom receptor may be either Co(bpy)2 or Co(bpy) as shown in Scheme II. 

With strong metal-metal interactions across a bridging ligand, the valence redox orbitals are delocalized molecular orbitals both metal and ligand in character. In mixed-valence compounds, different, discrete oxidation states do not exist since the site of oxidation is delocalized. Strongly coupled systems are like metal-metal bonds in that their electronic and chemical properties are significantly modified from those of related monomeric complexes. As with metal-metal bonds, such compounds can have an extensive multiple oxidation state chemistry based on delocalized molecular orbitals. 

Cyclic voltammetry demonstrated that the pyrazine-bridged dimer Ru3(pyz)Ru3 has an extensive oxidation state chemistry, and that the cluster-cluster mixed-valence ions [Ru3(pyz)Ru3] and [Ru3(pyz)Ru3] are discrete species in solution. Ruthenium ESCA data for the perchlorate salt of the +1 ion reveal the presence of distinct cluster units, Ru3 (pyz)Ru3 and therefore indicate that intercluster interactions across pyrazine are weak. However, the electrochemical data reveal that the extent of cluster-cluster interaction increases as the electron content of the system increases. The synthetic chemistry here is very promising, and we should be able to prepare complex two-dimensional systems including polymers and to investigate cluster-cluster and metal ion-cluster interactions. 

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