Oxidation state ambiguities

The very close vicinity of the abovementioned 5-values, and the surprising similarity of the optical spectra of Os(OEP)NO(OMe) [31c] and Os(OEP)N(OMe) [25c] are nice examples of the oxidation state ambiguity typical for the nitrosyl compounds the trivial representation of [25c] as a nitridomethoxoosmium(VI) derivative could justify the formulation of [31c] as a derivative of N03e and OsVI instead of NO and Os11. On the other hand, our preference to regard [57c] as a NO derivative (66) could imply the absurd view of [25c] containing a N ion stabilized by coordination. 

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. 

Instead, any one given complex has a structure in a range between 2.18a or 2.18b as extremes. The oxidation state ambiguity can become... 

Intermediate between the two extremes are carbenes, such as L M=C(Hal)2, the halide being intermediate in iv-donor strength between -H and -OMe neither model is satisfactory in this intermediate zone and we encounter another L/X2 oxidation state ambiguity. 

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. 

Although Re Hv(dppe) is classical, ReH7(P p-tol) l 3)2 has a ReH5(H2)L2 structure with a stretched H-H distance (1.357 A vs. the usual 0.8-1.0 A for a standard H2 complex), making the oxidation state ambiguous because the structure lies between Re(V) and (VII). As these examples show, polyhydrides often have coordination numbers in excess of 6, a consequence of the small size of the hydride ligand. 

When a solution of Pu(VI) is subjected to the 24 hr. irradiation the bulk of the Pu has been reduced to the Pu(III) oxidation state. There exists the ambiguity that residence on the column may cause some reduction of Pu(VI) Pu(V) and... 

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. 

In characterizing the aims, methods, and values of chemistry, some chemists recently have stressed the irreducibility of chemistry to physics. Hoffmann, for instance, in reflecting on the question, has written that "most of the useful concepts of chemistry (for the chemist aromaticity, the concept of a functional group, steric effects, and strain) are imprecise. When reduced to physics they tend to disappear."40 Further, it is the very ambiguity of some chemical concepts, like "oxidation state," that appeals to chemists. For chemists, the oxidation state of an atom varies according to its role in a molecule, appearing to vary from, perhaps, + 3 to -2 for an atom. Physicists are uneasy with this "elastic" heuristic device.41... 

Once the basic structure has been determined, bond valences can be used to resolve a number of problems of interpretation. Diffraction experiments can identify the location of each atom, but cannot identify its oxidation state. In most structures the oxidation state is determined by the requirements of electroneutrality (Rule 11.1), but in some structures more than one assignment is possible. Bond valence sums can usually resolve this ambiguity. 

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... 

---