Metals Oxidation states, ambiguities

For a lucid discussion of oxidation state ambiguities, see Crabtree, R. H. In The Organometallic Chemistry of the Transition Metals, 4th ed. Wiley Hoboken, 2005 Chapter 2, pp 29-41. 

The same oxidation state ambiguity that we have seen several times before also operates here. Equation 6.54 shows that if the alkenes are considered to be in the metalacyclopropane (X2 or form), the coupling reaction proceeds with formal reduction at the metal and resembles a reductive elimination of two alkyl groups. Parkin has a case of a reductive coupling in Eq. 6.55,... 

Structures 11.4 and 11.5 also show how oxidation states are assigned differently for the two types. Binding of a Fischer (singlet) carbene does not alter the oxidation state of the metal, but as an X2 diyl ligand, a Schrock carbene is considered to increase the oxidation state of the metal by two units. This creates another oxidation state ambiguity in intermediate cases where the bonding is not clearly classifiable as Fi.scher or Schrock. 

The Os complex of Eq. 11.31 contains a carbene with character intermediate between the Fischer and Schrock extremes because it reacts both with electrophiles such as SO2 (Eq. 11.31a) or H+ and with nucleophiles such as CO (Eq. 11.31b) or CNR. This is consistent with our bonding picture the osmium has ir-donor (Cl) as well as -ir-acceptor (NO) ligands, the metal is in an intermediate oxidation state (Os(II) if we count the carbene as L, Os(IV) if X2), and the carbene carbon has non-TT-donor substituents (H). Such carbenes cannot be securely classed as either Fischer or Schrock forms, leading to an oxidation state ambiguity, since the convention differs for the two forms. 

Older texts often employ an alternative nomenclature in which the suffixes -ous and -ic are encountered. In general, these labels only apply to the most common oxidation states of the metals, -ic referring to the higher oxidation state and -ous to the lower. Using this nomenclature, copper(ii) is referred to as cupric and copper(i) as cuprous. The system works well if there are only two common oxidation states for a metal ion, but if there are more, the scheme becomes either ambiguous or unwieldy as a variety of prefixes are added. 

The well-known ambiguities in limiting descriptions of metal and ligand oxidation states are summarized below. 

When Or is bound to a transition metal its formal oxidation state and that of the metal are ambiguous. In the compounds discussed above the MoVI-peroxide formulation is used. However, in the absence of structural data an MoIV-dioxygen or Mov-superoxide formulation is possible. The hypothetical reaction of MoIV and 02 can be considered an oxidative addition, wherein the extent of charge transfer determines the proper formulation. In the complexes discussed here the O—O distance lies in the range 1.44 to 1.55 A. Comparison of these... 

The foregoing classification is not without ambiguity. For example, it is common practice to call the reaction A - B +C° (see Fig. 6-1) induced by decreasing the temperature a phase transformation. The similar (peritectoid) reaction C = a+fi (Fig. 12-2) induced by a temperature increase, however, is named a decomposition reaction. In addition, the isothermal reaction AO = A+j02, which occurs if the intensive variable fio2 is decreased so that AO decomposes, is called a metal oxide reduction. It is thus categorized as a genuine heterogeneous solid state reaction (the... 

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