Dioxygen complexes structural data

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... 

Several conclusions may be drawn from this diagram concerning the geometry of dioxygen complexes. Complexes of the type MXY 4 should exhibit the side-on rj2 geometry. Many structures of this type are known, e.g. Ti(0EP)(02),13 MoOF4(02)2 14 and V0(H20)(dipic)(02) , 1S The next known dioxygen complex occurs at MXY 7. This is the compound Mn(TPP)(02).l< No structural data are available. 

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. 

Table 16.2.2. Structural data of dioxygen metal complexes...

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. 

The phthalocyanine complexes of manganese have been studied for many years ° The most recent work has established the formation of an rf dioxygen complex on oxygenation of [Mn(Pc)] in N,N-dimethylacetamide solution and a similar result has been obtained for a tetra-sulphonated phthalocyanine derivative The porphyrin complex Mn(TPP)02 was assigned an rf structure on the basis of E.P.R. data and this assignment has recently been supported by infra-red data using 02 Schiff base complexes of Mn(II) were reported to form dinuclear rf complexes on oxygenation in... 

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". 

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. 

Molecular oxygen adducts of transition metal complexes arc of interest and importance to catalytic processes and commercial oxidation processes, as well as being intermediates in oxidation reactions. Vaska " has reviewed the nature of dioxygen bound to transition metal complexes. All known iridium dioxygen complexes possess the peroxo structure (140). Experimental data reveal that the formation of covalent Ir—(O2) bonds on dioxygen addition to IrL, is accompanied by extensive redistribution of electrons, and the electron transfer is from the iridium to dioxygen. SCF-X -SW calculations on [Ir(02)(Ph3)4] and [Ir(Ph3)4] " indicate peroxo -metal bonding. ... 

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