Qualitative Models of Dioxygen Binding

The bonding in peroxychromates was initially discussed in completely different terms. Tuck and Walters considered the primary interaction to be that of the r bonding orbitals 

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 . 

Apart from the unambiguous and useful classification of the dinuclear cobalt complexes as /i-peroxo or /i-superoxo species, we shall not use formal oxidation states frequently in this review, and we limit the discussion here to the following points  

Most recent authors have discussed the bonding in dioxygen complexes in terms of molecular orbital theory. Thus Drago in his spin-pairing model for 77 complexes of cobalt proposes a M.O. diagram arising fi om the interaction of one jtg orbital of dioxy- 

Overlap (i) is that proposed by Drago in his spin-pairing model and overlap (iii) that proposed by Stomberg for the peroxychromates. On overlap grounds alone the if complex (overlap (iii)) will clearly be more stable, and, indeed, rf complexes are the 

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