The Dioxygen Ligand

The dioxygen ligand in mononuclear group VIII transition metal complexes. J. S. Valentine, Chem. Rev., 1973, 73, 235-245 (101). 

This review is particularly concerned with the bonding modes of the dioxygen ligand, and the factors affecting which of the various possible orientations it assumes in any particular complex. We shall therefore consider all previous work, from the earliest work on synthetic oxygen carriers, right up to the most recent studies on picket-fence and other synthetic porphyrins. All work concerned with an irreversible system will illustrate some principle appertaining to reversibility. On this basis we shall then attempt to provide a unified rationale for ... 

Type 1. Bridging adducts (see Fig. 2), where the dioxygen ligand is bridged between two metal atoms intermolecularly. Here, the metal dioxygen ratio is invariably 2 1. 

B. Theoretical Considerations Concerning the Orientation and Electronic Structure of the Dioxygen Ligand. 

We shall return to the discussion of the orientation and electronic structure of the dioxygen ligand during the next section which is primarily concerned with recent advances in the synthesis and physical characterization (particularly X-ray analysis) of oxygencarrying metal porphyrin model compounds. 

In these reactions triphenylphosphine was liberated, and the IR spectrum of the product showed that the dioxygen ligand was no longer present (absence of the strong 850 cm 1 bond). X-ray analyses showed a Rh P Cl ratio of 1 1 1 in these products. The presence of amine or amide ligands in the catalytic species was verified by IR, PMR, and elemental analysis. Despite this instrumental and analytical data, no reasonable formula can be proposed for any of these species. 

This strategy has deliberately avoided the superoxo/peroxo dichotomy. The extent of electron transfer to dioxygen in these adducts is difficult to assess and is a subject which remains controversial. However, it is convenient to divide the wealth of metal-dioxygen complexes into superoxo-type and peroxo-type upon the basis of structural data from the point of view of ease of discussion. We have done this purely on the basis of bond lengths and IR data and do not intend this to be interpreted as a measurement of the extent of electron transfer from the metal to the dioxygen ligand. 

These molecules, which also reversibly bind the dioxygen ligand, may help in unraveling the mysteries of oxygen-binding and activation. 2,s... 

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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