Oxidation states ligand characteristics

Low oxidation states - An important characteristic of transition metal chemistry is the formation of compounds with low (often zero or negative) oxidation states. This has little parallel outside the transition elements. Such complexes are frequently associated with ligands like carbon monoxide or alkenes. Compounds analogous to Fe(CO)s, [Ni(cod)2] (cod = 1,4-cyclooctadiene) or [Pt(PPh3]3] are very rarely encountered outside the transition-metal block. The study of the low oxidation compounds is included within organometallic chemistry. We comment about the nature of the bonding in such compounds in Chapter 6. 

The ability of the nitrido ligand to stabilize the Tc(VI) oxidation state is apparent in the stability of the [TcN]3+ core to hydrolysis and disproportionation. This is in marked contrast to the [TcviO]4+ core, which is highly susceptible to oxidation and to disproportionation according to 3TcVI — TcIV + 2Tcvn and is not readily stabilized by coordination [4]. A characteristic feature is the formation of dimeric [NTcOTcN]4+ and [NTc(/i-0)2TcN]2+ complexes, which have no ana-... 

As noted above, biouptake involves a series of elementary processes that take place in the external medium, in the interphasial region, and within the cell itself. One of the most important characteristics of the medium is the chemical speciation of the bioactive element or compound under consideration. Speci-ation not only includes complexation of metal ions by various types of ligands, but also the distribution over different oxidation states, e.g. Fe(II) and Fe(III), and protonation/deprotonation of organic and inorganic acids of intermediate strength. The relationship between speciation and the direct or indirect bioavailability1 of certain species has received a lot of recent attention. 

As emphasized by this latter study, it has been relatively straightforward to identify myoglobin variants that are five-coordinate in both oxidation states, but it has been far more difficult to identify variants or derivatives that are six-coordinate in both oxidation states. Myoglobins with this characteristic would have the potential to provide considerable insight into the role of various types of axial ligand in regulating the electron transfer reactivity of cytochromes and other types... 

Complexes containing rhenium in the oxidation state +IV are comparatively rare. There is no extended chemistry in aqueous media and many rhenium(IV) complexes tend to hydrolyze when exposed to water. The stabilization of rhenium(IV) centers requires a well-balanced donor-acceptor behavior of the ligands and, thus, none of the classical rr-donor ligands such as O or TT-acceptors such as carbonyls or nitrosyls are characteristic for this oxidation state... 

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