Structural Classification of Dioxygen Complexes

Structure type Structural designation Vaska classification Example 

The rj rj structure with a sideways bound dioxygen bridging two metal atoms has been suggested for the complex [ Rh(diene) 202] and a crystal structure showing this geometry has been reported for the uranium complex [(1)02013)202] and a complex of La The rf structure (Fig. 2) is known only for [RhCl(02)(PPh3)2]2  

The only completely unambiguous method of structure determination has proved to be X-ray diffraction. Vaska noted however that the stretching frequencies attributed to the 0-0 vibration were closely related to the structural type . Type I complexes show 0-0 stretching vibrations around 1125 cm and type II around 860 cm . This sharp difference enables the 0-0 stretching frequency as measured by infra-red or Raman spectroscopy to be used to assign the structure type, provided (as is usually the case) the formation of a dinuclear species can be confirmed or excluded by other means. 

If X-ray diffraction gives an unambiguous description of the structure, it should nevertheless be noted that the accurate determination of bond lengths and angles for the coordinated dioxygen is not always easy. When the dioxygen is bonded closely to a very 

Structural investigations using methods other than X-ray diffraction have also provided much useful information, and in the remainder of this section we review some of the results obtained from such investigations. 

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Figure 90 Structural classification of dioxygen complexes.11641145 Yaska classification superoxide 1(a) and 11(a) ...

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. 

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. 

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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