Dioxygen complexes structural classification

Figure 90 Structural dassiflcation of dioxygen complexes. Vaska classification superoxide 1(a) and 11(a) ...

Further to section III(C), we shall see that type I dioxygen complexes can be formed in solution via intermediates of a type 11(F) structure. Consequently, sections IV( ) and IV(F) will tend to overlap in places. The dioxygen adducts of some new synthetic metal porphyrins belong to the 11(F) classification. However, a discussion of the dioxygen adducts of naturally occurring and synthetic metal porphyrins is postponed until section V. 

Single-crystal X-ray structures have now been determined for a large number of cobalt-dioxygen complexes and comparisons of the observed O—O bond lengths with those of superoxide anion or peroxide dianion (O—O separation = 128pm, K02 149 pm, Na202) also form a reliable guide to classification. 

We have already discussed the use of spectroscopic data for the structural classification of dioxygen complexes in section C and in this sub-section we shall be concerned with the information on electronic structure to be obtained from spectra. 

There have been numerous x-ray structure determinations of dioxygen complexes. Tables VI - X give a list of the most important structural data for dioxygen complexes grouped according to Vaska s classification scheme. 

The stretching frequencies attributed to the 0-0 vibration are closely related to the structural type. Type I complexes show 0-0 stretching vibrations around 1125 cm-1 and type II around 860 cm-1. This sharp difference enables the O-O stretching frequency as measured by infrared or Raman spectroscopy to be used for structure type classification. Table 16.2.2 lists the structural data of dioxygen metal complexes. 

Werner correctly identified the dinuclear complexes that Vaska classifies as type II b as r-peroxo complexes of two Co(III) ions, but it was only with the advent of modem physical techniques that it was possible to show by E.P.R. that the unpaired electron in Vaska type I b complexes such as [(H3N)5Co02Co(NH3)5] is localised on the dioxygen ligand , leading to their classification as <-superoxo complexes. X-ray structural data (Table 5) show that the 0-0 bond lengths in type Ib complexes are significantly shorter (and closer to the value for free Of) than those in type lib complexes which lie close to the values obtained for Ol". 

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