Oxidation states organometallics

The reactions learned in Chapter 20 have proven extremely useful in organic synthesis. Oxidation and reduction reactions interconvert two functional groups that differ in oxidation state. Organometallic reagents form new carbon-carbon bonds. 

Because of these problems, chemists have tried to use low oxidation state organometallic precursors for the preparation of the advanced heteroge-... 

A low oxidation state organometallic complex contains tt-acceptor ligands and the metal centre tends to acquire 18 electrons in its valence shell (the 18-electron rule), thus filling the valence orbitals, e.g. Cr in Cr(CO)g, Fe in Fe(CO)5, and Ni in Ni(CO)4. 

In Section 20.4, we apphed molecular orbital theory to octahedral complexes containing w-acceptor ligands and gave a rationale for the fact that low oxidation state organometallic complexes tend to obey the 18-electron rule. This rule often breaks down for early and late J-block metals as examples later in the chapter show 16-electron complexes are common for e.g. Rh(I), Ir(I), Pd(0) and Pt(0). The majority... 

There will be circumstances other than those I have described here in which "high oxidation state" organometallic chemistry of rhenium in a catalytic reaction will be viable, although it is becoming clear that the balance necessary to achieve this feat is more difficult to maintain as one moves to the right in the transition metal series, and that some of the d rhenium chemistry in fact may look like chemistry of dP osmium species. On this basis it would seem unlikely that the principles that have been used to prepare Re(VII) alkylidyne and alkylidene complexes (a hydrogen migration reactions) can be extended further (to technetium, or especially osmium or ruthenium), at least in a routine fashion. 

FIGURE 15.2 Some high-oxidation-state organometallic chemistry of the Cp Re... 

Although the 18-electron rule is quite widely obeyed for low oxidation state organometallic compotmds containing TT-acceptor ligands, it is useless for higher oxidation state metals. This is clear from examples of octahedral complexes cited in Section 19.7, and can be rationalized in terms of the... 

Paramagnetic and high-oxidation-state organometallics have been relatively neglected because they are harder to study. 

For the purpose of assigning oxidation states, organometallic ligands such as cyclopentadienyl, alkyl, and allyl are often considered to be anionic. Thus, the formal oxidation states of the metals in Ti(Cp), RjTa (R = Bu CH ), and [Ni(allyl)J are Ti, Ta, and Ni + provided the ligands are treated as Cp, R, and. With alkylidene, alkylidyne, and NHC carbenes, the assignment of the oxidation state is a little more complicated. As discussed later, it is best understood in conjunction with the electron counting scheme. 

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