M=O bond

There has been growing interest in the detailed structure and reaction chemistry of monomeric forms of two-coordinate derivatives of Ge , Sn and Pb" since the first examples were unequivocally established in 1980. Thus, treatment of the corresponding chlorides MCI2 with lithium di-tert-butyl phenoxide derivatives in thf affords a series of yellow (Ge , Sn ) and red (Pb ) compounds M(OAr)2 in high yield.The O-M-O bond angle in M(OC6H2Me-4-Bu2-2,6)2 was 92° for Ge and 89° for Sn. Similar reactions... 

Recall that the unit cell in the spinels comprises AgBi6032. In the normal structure, there are 16 B ions in octahedral sites and 8 A ions in tetrahedral ones. That corresponds to 96 octahedral B-0 bonds and 32 tetrahedral A-0 bonds or 128 bonds in all. In the inverse structure, we have 8 B ions in tetrahedral sites, 8 B ions in octahedral ones, and 8 A ions in octahedral sites. This corresponds to 48 octahedral B-O bonds, 32 tetrahedral B-O bonds and 48 octahedral A-O bonds or once again, 128 bonds in all. So the total number of M-O bonds, different types to be sure, is the same in both normal and inverse spinel structures. We could spend quite some time estimating the different bond energies of A-0 and B-O or of octahedral versus tetrahedral, but that would undoubtedly involve a lot of guesswork. We can at least observe that the bond count factor difference between the spinel... 

The kinetics of H-O recombination is very important in the reforming reaction of methane to produce CO and H2. When more weakly bonded O js recombines with Hads (preferred on Pt), the main product next to CO will be H2. On planar Rh with a stronger M-O bond interaction, this reaction is suppressed and therefore H2 is the main product [23]. Clearly this selectivity will be dramatically affected by the presence of surface steps. 

It is observed that higher potential values for the adatom redox process are correlated with a lower energy of the M—O bond, i.e., lower (less negative) enthalpy of formation of the adatom oxygenated species. In this regard, the discrepant behavior of Ge-Pt(lOO) may be related to the dilute nature of this adlayer, with a maximum coverage of only 0.25. 

O-M-O bond angles required. One can relate bite sizes (distances between the donor atoms) of strain-free chelate rings of different sizes to the dimensions of the chair form of the cyclohexane ring (39), which has the minimum steric strain possible for a cycloalkane. The bite size of strain-free four- and six-membered chelate rings both correspond to... 

Fig. 20. The variation in strain energy, V, for various conformations of [M(18-crown-6)]n+ complex, as a function of strain-free M-0 bond length. The M—O bond lengths of various metal ions are indicated on the M-0 bond length axis. The curves are for the planar D3d (+-+-+-), half-buckled (+-+--), and buckled (++-+ +-) conformers shown in Fig. 21, and for the complex of the open chain complex of pentaethylene glycol. The calculations were carried as described in the text, and in Refs. (4 and 60). Redrawn after Ref. (60). 

Attempts were made at explaining the trends in reactivity through the use of both an electron-transfer model85 and a resonance interaction model,86,87 but without success. It seems that the trends in reactivity on a fine scale cannot be easily explained by such simple models, but instead depend on a multitude of factors, which may include the ionization potential of the metal, the electron affinity of the oxidant molecule, the energy gap between dns2 and dn+1s1 states, the M-O bond strength, and the thermodynamics of the reaction.57-81... 

In addition, the values of percent reduction listed in Table I increase roughly with increasing the it bond energy, although the results for M—O bond compounds are somewhat exceptional. It should be noted that the values except for C=0, C=S, and C=Se are similar to each other, being 8-10% irrespective of the atomic numbers of constitutional elements. 

Most of these reactions are promoted with an inorganic base such as KOH, NaOH, or K2C03 as an essential co-catalyst. For reaction without alkaline bases see Mizushima et al.137 Many of these complexes contain a chloride ligand, which is easily displaced by an alkoxide displacement/ /3-hydride elimination sequence in the presence of a base to remove HC1 formed (Scheme 25). In contrast, cationic LnM+ systems add the alcohol by formation of M—O bond, with the base... 

The lanthanide oxide cations [LnO]+ and the bare lanthanide ions Ln+ react differently with butadiene (162). Some bare Ln+ ions (La, Ce, Pr, Gd) activate butadiene but their oxide cations are inert toward butadiene. The lanthanides with weak M-O bonds, EuO and YbO, react by oxygen transfer to the butadiene. The oxide cations of Dy, Ho, Er, and Tm activate butadiene, whereas the bare metals of these lanthanides are unreactive with butadiene. The [HoO]+ ion has been studied in detail and is able to polymerize butadiene the mechanism of this reaction has been discussed. 

Ligand displacement reactions for [MX]+ and [MX2]+ ions may be used as a measure of ligand strength for a wide variety of donor molecules. The M-O bond in the [MO]+ ion varies from very strong for the groups 3 (IIIB), 4 (IVB), and 5 (VB) metals to weak in the later transition metals. 

Of special importance is the fact that upon protonation, the M = O bonds along the apical 0 = M-0H and 0 = M-0H2 axes are shortened significantly, coinciding with a weakening of the protonated M-0 bond. This further results in an increase in distortion as observed from the displacement (A) of the metal center from the equatorial plane formed by the four cyano carbon atoms (see Table I). 

Diastereomerically pure iridium complexes of the formula [(ri -C5Me5)lr (/ )-Pro-phos (activated alkene)](SbF6)2 (activated alkene = enal, methacrylonitrile) are active, and selective catalysts for the DCR between one point binding activated alkenes and nitrones. Enals coordinate to the metal in a completely diastereoselec-tive way with a restricted geometry. From the point of view of the selectivity, a key point in enal coordination is the establishment of CH/n-attractive interactions between the CHO aldehyde proton and one (f )-Prophos phenyl group. This interaction fixes the methacrolein rotamer around the M-O bonds and renders the system enantioselective. 

Fe +/Fe + couple is at 2.8—3.5 V in these struc-tures °), namely, the so-called inductive effect of the phosphate groups. The covalent P—O bonds of the phosphate groups will presumably increase the ionic character of the Li—O and M—O bonds in these compounds,reducing the extent of overlap and thus hyperfine interactions in these materials. 

Figure 1.10 Degree of covalence (/J.) vs bond strength (S /) in M-O bonds for cations with 18, 36, and 54 electrons. Values are in valence units and scales are logarithmic. Reprinted from Brown and Shannon (1973), with kind permission from the International Union of Crystallography.

Raman and i.r. spectra of [MgOig] - (M = Nb or Ta) salts have been determined and normal-co-ordinate analyses suggest that the ratio of the force constants of the terminal, bridging, and central M—O bonds is close to 8 4 1. 

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