Transition metal complexes multiple oxidation states

It should be recalled that one of the principal properties of transition metals is their aptitude to accede to multiple oxidation states. Thus, the main scope of an electrochemical study is to ascertain whether a metal complex, prepared in a certain oxidation state, is stable also in different oxidation states, or whether the lifetimes of these oxidation states are too short to observe stable products. Whenever stable oxidation states are identified, the chemist might be able to prepare and fully characterize these new complexes. 

Two classes of catalysts account for most contemporary research. The first class includes transition-metal nanoparticles (e.g., Pd, Pt), their oxides (e.g., RUO2), and bimetallic materials (e.g., Pt/Ni, Pt/Ru) [104,132-134]. The second class, usually referred to as molecular catalysts, includes all transition-metal complexes, such as metalloporphyrins, in which the metal centers can assume multiple oxidation states [ 135 -137]. Previous studies have not only yielded a wealth of information about the preparation and catalytic properties of these materials, but they have also revealed shortcomings where further research is needed. Here we summarize the main barriers to progress in the field of metal-particle-based catalysis and discuss how dendrimer-encapsulated metal nanoparticles might provide a means for addressing some of the problems. 

Discuss the role of transition-metal complexes in biology. Consider such aspects as their absorption of light, the existence of many different structures, and the possibility of multiple oxidation states. 

The chemistry of the transition elements is rich and varied. Multiple oxidation states and coordination numbers, together with complex spectroscopic and magnetic properties, afford a plethora of structures and reactivities. Transition Metal (TM) species have therefore found widespread application both by nature and by man. 

The electrochemistry of large numbers of organic species is well established, and the occurrence of multiple oxidation states is commonplace for transition metal compounds. By comparison, the redox reactions of organotransition metal complexes are relatively unexplored. 

Because of its large size and accessibility to multiple oxidation states, uranium is capable of unprecedented reactivity and beautiful coordination complexes that caimot be achieved with transition metals or lanthanides. The exciting products highlighted here demonstrate that we have only just begim to learn the capabilities of uraniiun, and that continuous studies will be needed to determine the full realm of possibiUties. From activation of small molecules to unique magnetic properties, uranium offers a synthetic and spectroscopic challenge to coordination chemists of the future. 

Transition elements make up the lower left-central portion of the periodic table of the elements, bridging the s-block elements at the left and the p-block elements at the right. Five properties are commonly attributed to transition elements (1) they are all metals, (2) many of them form compounds involving a variety of oxidation states, (3) many of them form compounds and aqueous solutions that are colored, (4) many of them form complex ions, and (5) many of them form compounds exhibiting paramagnetic behavior. This experiment involves multiple oxidation states of several transition elements a variety of colored solutions, and several complex ions involving a transition metal cation and oxygen. 

The zinc ion is colourless. Zinc shows some catalytic properties and does form complex ions, although this property is not unique to transition metals. Scandium shows some similarities to zinc but is classified as a transition element because it can exist in multiple oxidation states +3 (common), +2 (rare) and +1 (rare). 

An imusual ability to adsorb gaseous species makes some transition metals, such as Ni and Pt, good heterogeneous catalysts. The possibility of multiple oxidation states seems to account for the ability of some transition metal ions to serve as catalysts in certain oxidation-reduction reactions. In still other types of catalysis, complex-ion formation may play an important role. As we saw in a limited way in Chapter 18 and will explore more fully in Chapter 24, complex-ion formation is a particularly distinctive feature of transition metal chemistry. 

Apart from the hardness and softness, two reactivity-related features need to be pointed out. First, iron salts (like most transition metal salts) can operate as bifunctional Lewis acids activating either (or both) carbon-carbon multiple bonds via 71-binding or (and) heteroatoms via a-complexes. However, a lower oxidation state of the catalyst increases the relative strength of coordination to the carbon-carbon multiple bonds (Scheme 1). 

---