O2 complexes

The diamagnetic nature of most O2 complexes might facilitate reactions to form diamagnetic products which would otherwise be hindered by the requirement of spin conservation ... 

The interaction of small, well defined, rhodium clusters, Rh and Rhs, with O2 has been investigated (220) by matrix infrared, and UV-visible, spectroscopy, coupled with metal/02 concentration studies, warm-up experiments, and isotopic oxygen studies. A number of binuclear O2 complexes were identified, with stoichiometries Rh2(02)n, n = 1-4. In addition, a trinuclear species Rhs(02)m, m = 2 or 6, was identified. The infrared data for these complexes, as well as for the mononuclear complexes Rh(02)x, = 1-2 (229), are summarized in Table XI. Metal-concentration plots that led to the determination of... 

The metal-atom reactions of Cu (98), Ag (134), and Au (135) with O provided interesting results, especially when these were compared with the results from the nickel triad (137). As shown by standard matrix-techniques, Ag forms two O2 complexes that are best formulated as Ag 02 and Ag+Oj, based on the absence of visible absorptions and the similarity of the IR spectra to those of Cs" 02 and Cs Oj (3a,b). The UV absorptions for Ag(02) and AgiOJ, at 275 and 290 nm, respectively, could be associated with the O2 and O4 anions. The shifts in the IR spectra on going from Ag(02) to Ag(04) also argue against an (02)Ag(02) formulation for the latter complex, being in the opposite sense to those observed for Pd(02)ian[Pg.139]

In the CP-O2 complex, the CP surface is an electron density donor. For example, in the case of PANI, the bond orders in adsorbed 02 molecules decrease by about 30%, and the bond lengths L increase by about 24%. Thus, the adsorbed 02 molecules have a fairly high degree of activation and can readily interact with the protons. Hence, quantum-chemical analysis confirms the mechanism of O2 electroreduction and gives possibility to understand the reasons of PANI catalytic activity. 

Type 1 intrazeolite photooxygenation of alkenes has been also reported to give mainly allylic hydroperoxides (Scheme 42). In this process, the charge transfer band of the alkene—O2 complex within Na-Y was irradiated to form the alkene radical cation and superoxide ion. The radical ion pair in turn gives the allylic hydroperoxides via an allylic radical intermediate. On the other hand, for the Type II pathway, singlet molecular oxygen ( O2) is produced by energy transfer from the triplet excited state of a photosensitizer to 02. 

There would appear to be two distinct modes of reactivity of early transition metal alkyls with O2. When the metal is not in its highest oxidation state, an O2 complex of variable stability may form, and its subsequent reactivity may or may not involve the metal-carbon bond. The formation of remarkable stable 0x0 alkyls is an example of this pathway. In contrast, d°-alkyls react with O2 by a radical chain mechanism that invariable leads to formation of alkoxide complexes labile alkylperoxo ligands are clearly imphcated as intermediates in these reactions. 

Finally, a well-characterized 02-insertion transforms Tp Cr(02)Ph into the paramagnetic 0x0 alkoxide Tp Cr(0)0Ph, see Scheme 12 [4]. This reaction proceeds below room temperature, and the starting material has only been characterized by in-situ IR spectroscopy. However, analogous O2 complexes were isolated and characterized by X-ray crystallography, so there can be little doubt about its assignment as a side-on bonded Cr(IH) superoxo complex. 

Carbon monoxide has 14 electrons, which pair to give a net spin of zero. Carbon monoxide complexes of transition metals, like oxygen complexes, cannot convert an even electron system to an odd electron system. In the case of iron, CO usually binds only to ferrous ions, which have six 3d electrons. As a consequence, CO complexes and O2 complexes with iron-containing proteins are generally not detectable by EPR. 

The 630 nm band, on the other hand, shows no significant intensity variation when the foreign-gas density is varied an oxygen density squared behavior is observed regardless of the presence and concentration of the admixtures. This band arises from simultaneous transitions of both molecules of the O2-O2 complex. In this case, an O2 molecule cannot be replaced by a foreign gas particle without losing the band, or perhaps shifting it to a different part of the spectrum. 

Few synthetic studies have appeared in the literature that deal with CS3 or CS O2- complexes. 

The structure of the complexes formed when gold clusters react with oxygen is not always certain, and in particular it is not certain whether the oxygen molecule is dissociated or not.3 Clearly if it is to dissociate, the transfer of charge from the cluster to the molecule must be the first step, but what must happen thereafter if dissociation into atoms is to occur is far from obvious. Some light is cast on this problem by the observation of the vibrational fine structure in the ultraviolet photoelectron spectrum (UPS) of the Au —O2 and Au)C -O2 complexes.8,9 Subsidiary peaks at 179 and 152 meV compare with those found elsewhere for the O2 ion at 80-120 meV and for the 0 ion at 135-150 meV (ImeV = 8.06 cm"1) this suggests... 

Oxygen molecules react with Au - OH clusters only when n is even.12 It is worth noting that an AU-O2 complex has been detected in matrix isolation chemistry. When gold atoms and oxygen were co-condensed with a rare gas at extremely low temperature, a green complex was formed in which the oxygen was bonded sideways on. 

The formation of the O2 complex usually involves some transfer of electron density from the metal to the O2 ligand. In other words the cobalt is partially oxidized. This partial oxidation is facilitated by N-donor ligands that compensate for the electron withdrawal by O2. 

O2-N2 systems, although in considerable less detail. Previous results on the N2-N2 system [44] have been reanalyzed, and a characterization of the N2-O2 complex has been presented a subsequent paper [10]. These results indicate that most of the bonding in the dimers comes from van der Waals (repulsion - - dispersion) and electrostatic (permanent quadrupole-permanent quadrupole) forces. On the other hand, chemical (spin-spin) contributions are not negligible for O2-O2, which is an open-shell-open-shell system [4,5]. Therefore the geometrical properties of the three dimers have been found to show interesting differences, to be seen in the next paragraph. 

The AE values of the Zn —02" complexes were evaluated from deviation of the g22 values from the free-spin value (69). The AE values of the O2 complex with [Zn°(Melm(Me)2)] + (0.87 eV) and [Zn (MeIm(Py)2)] + (0.85 eV) are significantly larger than those of O2 complexes with other divalent metal ions [Mg(ll)... 

O2 complexes have been reported trans indicates that the two metal centers are diagonally located on opposite sides of the O2 ligand and cis sigitifies that the metal centers are on the same side of the O2 hgand). Therefore, superoxo or peroxo is also stated to fully describe the metal dioxygen interaction in a complex. 

These are often referred to as traits-fx,-l,2 complexes. The snperoxo trans-fi-ri ri -O2 complexes typically give u(OD) in the range 1000 to 1100 cm and OD bond distances between 1.24 and 1.36A and the dioxygen moiety is generally planar (dihedral angle, = 180°). Examples are the tetrameric snperoxo complex [02 Co(F salen)(H20) 2]2 [F salen = Al,Al -ethylenebis(3-fluorosaUcylideneiminato)] that has a dihedral angle of 122° and an 0-0 distance of 1.31 (3) A and the Ni 2-bis(superoxo)... 

In the synthetic iron porphyrin, O2 affinity mainly depends on the strengths of the <7-donation from the lone pair of O2 to the heme-iron dz orbital and 7r-back donation from the d/r orbital on the iron to the Jt orbital of 2. To evaluate the O2 affinity and/or O2 binding dynamics in myoglobin and hemoglobin, O2 - protein interaction is a further important factor. For example, the O2 dissociation rate constant for oxymyoglobin is relatively smaller than those of O2 complexes... 

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