Side-on complexation

Secondly, there is an indication that metal(III)-peroxo side-on complexes are in general in equilibrium with corresponding metal(II)-superoxo end-on species. The position of such equilibrium could depend on various factors as structural and electronic properties of the porphyrin ligand, coordination of an axial ligand trans to peroxide/superoxide, solvent medium, temperature and involvement of coordinated peroxide/superoxide in possible hydrogen bonding or electrostatic interactions. These are interesting questions which should be addressed in future studies. 

The closely related lead analogue 81 shows essentially the same structural features, but the lead atom in 81 is less pyramidalized (355.7°, see Table XIV), which indicates weaker coordination. The NMR spectroscopic parameters for 81, such as " Pb NMR chemical shift and scalar Pb-C(sp) coupling constants (see Section III.B.3) clearly demonstrate however the side-on complexation of the plumbyl cation by the C = C triple bond in zwitterions 81. ... 

Fig. 13. Dehydration of methanol on a Brdnsted acid site (a) shows the side-on complex, (b) the transition state, and (c) the dissociative complex of surface methoxy and water. Reprinted with permission from Ref. 221. Copyright 1995 American Chemical Society.

For example, Allen et al. have recently isolated an iron carbonyl complex with a bicarbonate counterion (Fe(dmpe)2(C0)H)(HC03) with dmpe = 1,2 bis(dimethylphosphino)ethane, resulting from the iron(II)-mediated reductive disproportionation of C02 [104], although the reaction mechanism is still speculative. In a related system, Karsh et al. [9] had observed, during the reaction of C02 with Fe(PMe3)4, the formation of a side-on complex Fe(C02)(PMe3)4 and of a carbonyl carbonate species Fe(PMe3)3(C0)(C03). 

The dissociation energy, AE, of MH3+ (Jhb) to MH+ and H2 decreases down group 14 and the reaction becomes exothermic for M = Sn and Pb (Table 43). However, as this dissociation proceeds through the side-on complex intermediate, 114, it has to overcome a high activation barrier ( a) of ca 50 kcalmol-1461. The planar MH3+ (D31,) cations are thus kinetically stable even for M = Sn and Pb and are therefore predicted to be observable. The side-on complexes are less stable than the Z)3h isomer toward expulsion... 

FIGURE 38. Calculated geometries (at CCSD/TZ2P for M = Si497 and CCSD/TZP+f for M = Ge494) of the side-on complex of MH5 1 (124). Bond lengths in pm... 

The nature and the possible structures of Xj and X2 are indicated by their electronic spectra. The absorption maximum at 425 nm in benzene and the shape of the spectrum of Xl9 the first short-lived intermediate (Figure 15), closely resemble those of the spectrum (A.max at 422 nm in methylene chloride) of TPP Mn(IV) prepared independently (31, 32, 33, 36), possibly indicating a charge-transfer complex (Figure 17). This assumption of a loose side-on complex resulting from a two-electron transfer is consistent with the previous results of the electron spin resonance (ESR) spectrum observed for an oxygencarrying species, TPP Mn(II) or substituted TPP Mn(II) (37). The... 

Not only the basic XH4 neutral molecules, but also the XH3 cations exhibit different structural preferences. Again the carbon case is the simplest the methyl cation CH3 exists only as a single >3 symmetric isomer. Dissociation into H2 and CH is ca. 137 kcal mol endothermic. However, the CH3 PES does have a second minimum. This is a weakly bound (1.8 kcal mol ) side-on complex between the C-H hydrogen and H2. 

The side-on complexation mode of the metallocene to the alkyne function is easily recognized spectroscopically, since it leads to characteristic low-field shifts for the 13C resonances such that for complex 22, S is 227.7 (C,, ) and... 

The problems (and the most interesting features) found in the spectroscopy of ionic metal cyanides and isocyanides occur because the CN ligand is nearly spherical. The CN ligand thus has a low barrier-internal rotation in M+CN" molecules. The equilibrium structure in the ground state can be linear cyanide, MCN, linear isocyanide, MNC, or even a T-shaped side-on complex (e.g., Na+N-)- The molecular geometry is evidently determined by... 

In other complexes, N2 is bound both end-on and side-on. In some end-on/side-on complexes, each metal plays its customary role, with an alkali metal bound side-on and a late transition metal bound end-on." " In one very unusual compound, though, lithium atoms are at the ends of N2 and nickel coordinates to the side. In some homometallic clusters, N2 is bound end-on to some metals and side-on to others. One of the best-characterized examples of end-on/side-on binding is in [(NPN)Ta]2(N2) (2), " in which the unsymmetric binding of N2 activates it toward... 

It is only a small extrapolation to move from side-on complexed H2 to side-on complexed X— H, and Crabtree has commented on how these interactions are related. The name agostic is often used for the case of X = a suitably substituted carbon atom. There are also a number of examples of X = a suitably substituted silicon atorn. " The agostic interaction of a C—bond, (25), results in a low-frequency shift of the proton resonance (due to the development of hydride-like character) and substantial reduction in the one-bond coupling constant, V( C, H). This reduction can be 50% or more. Similarly, for X = SiRs, the one-bond, /( Si, H) value decreases. In the solid state one finds the C—H bond as a donor to the metal. There are many examples of this type of interaction. ... 

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