End-On Complexes

Summarizing, the in situ UV-Vis, XANES, and EXAFS studies of Bonino et al. [49] and of Prestipino et al. [50] on hydrated and anhydrous peroxo/hy-droperoxo complexes on crystalhne microporous and amorphous meso-porous titanosilicates have shown, for the first time, the equilibriiun between r] side-on and end-on complexes. The amount of water is the key factor in the equilibrium displacement. In this regard please note that, owing to the hydrophobic character of TS-1, substrates such as olefins are the dominant species in the channels. This fact assures a relatively local low concentration of water, which in turn guarantees a sufficient presence of the active end-on... 

The principal intermolecular donor-acceptor interactions of this weakly bound complex are found to be of 7tcc-OBrBr form (3 x 0.20 kcalmol-1), as illustrated in Fig. 5.41(b). A prominent feature of Br2 (and other heavy halogens) is the nearly pure-p character of the bromine bonding hybrid, resulting in a conspicuous backside lobe on the OBrBr antibond (see Fig. 5.41(b)) that is effective in end-on complexation to the pi-donor face of benzene. The unusually small energy separation between donor and acceptor NBOs,... 

Unfortunately, no electrochemical studies have been reported either for these complexes or for the possibly related ju-l,2-(end-on) complexes.31,32 The only electrochemical information available is related to a few binuclear Cu(I) complexes of general formula [Cu2(L )]2+ and their oxygenation products [Cu2(02)(L )]2 +, which, based on spectroscopic evidence, are likely to coordinate dioxygen in a p-rj rj -peroxo way. ... 

In C-coordinated complexes, the frequency splitting between the two v(CO) stretching modes is less than 400 cm-1, and in end-on complexes, the out-of-plane bending mode y(C=0), located in the 500-650cm-1 region, exhibits a preponderant lsO effect, in contrast with other coordination modes, where a larger 13C effect is observed (namely Ay(13C) = 10 to 20cm-1, versus Ay(180) = 5 cm-1). 

Equation (1) describes the chemisorption of O2 on a surface site A of a metal (Ma) in an acid medium, where coupled to a proton and an electron transfer leads to the formation of an adsorbed end-on complex HOO-Ma. The unstable intermediate subsequently dissociates into two adsorbed species, one adsorbing on A sites, 0-Ma and the OH species adsorbing on B sites, HO-Mb (Eq. 2). In the rest of the electroreduction steps, represented by Eq. (3), adsorbed O and OH are reduced to H2O and the water molecules are eventually desorbed from the metal surface. Actually, Eqs. (1)-(3) can also be used to interpret the ORR activity for Pt-skin surfaces. The electronic sfructures of surface Pt atoms are not identical due to the existence of 3d metal in the sublayers. Ma and Mb can be looked as two Pt surface sites with different activities for reactions (l)-(3). Ma site possess better performance for the formation of the OOH complex, and Mb site may enhance the dissociation of OOH. The overall ORR is thus facilitated by the skin sfructures. 

The activation of On by Co, Mn, and Mo porphyrins has been studied theoretically by Witko and coworkers [568]. In the case of the Co porphyrin, the active forms of the catalyst are found to include an end-on complex with dioxygen and the hydroperoxo form, but not an oxo O=Co(porph) complex because water will not easily dissociate. In the case of the Mn porphyrin, the side-on, hydroperoxo, and 0X0 types of ligands are possible. In the case of the Mo porphyrin, all forms, the side-on, end-on, hydroperoxo, and oxo forms, are possible. 

The side-on -coordinated N2 attains a nitride-like structure and is comparatively reactive. Thus, the formation of a side-on metal N2-complex in the nitrogenase is a reasonable way for N2-activation (for rev. see Ref.170)), but it should be mentioned that end-on -complexes have also been discussed as models171). Recent experiments showed170) that side-on -metal N2-complexes built up on reduced oxo-molybdenum in the valence state +4 and coordinated by cystein lead to diimid-for-mation followed by a rapid disproportion or decomposition of diimid and subsequent reduction of the generated hydrazine to NH3. The reaction sequence is given by ... 

In 3.22, one of the Zr d orbitals is unavailable for bonding due to the ancillary tridentate ligand, which positions the amine donor and the chloride to overlap effectively with that d orbital. This explanation is supported by MO calculations. X-Ray structures of related binuclear dinitrogen-Ti end-on complexes, e.g., 3.23 (Beydoun et al., 1992) support the conclusions of Fryzuk and coworkers. Compound 3.23 has (Z)-disposed neutral amine donors instead of ( )-disposed phosphine ligands in 3.22. Three other edge-on bi- and tetranuclear dinitrogen complexes are known (see Fryzuk et al., 1993, structures III-V). 

This dj —p delocalization more than compensates for the electron density donated to the metal (o — rfo bond) by the lone pair on one of the N-atoms or the C-atom of CO, respectively. This delocalization is called back donation or back bonding . An orbital representation for the bonding in end-on complexes of N2 is given in Figure 3-2. This description is supported by MO calculations performed by the groups of Hoffmann and Fukui (Hoffmann et al., 1977 Yamabe et al., 1980). 

We will discuss mainly these mononuclear end-on complexes, as they are more closely related to diazo compounds than the binuclear complexes. ... 

Nature of all substrates, aflSnity for N2ase, and reactions implicate transition metals in catalysis with end-on complexation for all substrates except C2H2. 

However, the direct attack of metal-bound dioxygen toward the carbon atom is not favorable energetically in an end-on complex Pt4CH2 (02), in which the sp -hybrid carbon atoms in the CH2 moiety are coordinately saturated. The formation of the lowest energy precursor complexl has an exothermicity of 33.4 kcal moP and the energy release may drive the hydrogen transfer from CH2 to metal atoms in complexl, which was verified to be the most favorable elementary step energetically in preliminary calculations. 

The IR spectra of the O-end-on complexes Mg <— O = C = O Mg [32] exhibit strong absorptions at 2 267 and 2 275 cm slightly red-shifted by 73 and 65 cm relative to free CO2, whereas side-on complexes usually exhibit shifts greater than 300 cm ... 

As anticipated in Sect. 1.6.3, NMR spectra (or else O-spectra) can contribute to structure investigation in solution and to gathering information about the behaviour of the CO2-TM complexes in the liquid phase. The multiplicity of peaks when P-Ugands are present on active metal nuclei can help one to understand the nature of bonding. The shift from free CO2 (124 ppm) may be an indication of how the C=0 is affected by coordination to a metal. So, for example, O-end-on complexes show C-resonance slightly up-field shifted (around ca. 121-110 ppm) with respect to the free cumulene, whereas side-on complexes usually show resonances down-... 

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