Bonding intermediate cases

This is supported by quantum chemical calculations, as well as by chemical experiments, which reveal for the tetrasilabicyclobutane structure a rather long (2.412 A) Si-Si central bond. Intermediate cases with a strong preference for a singlet ground state are exemplified by the boron systems 7, in which a singlet fragment (PR2 ) is flanked by two radical positions (BR ). 

In a sense the formation of t) -H2 complexes can be thought of as an intermediate stage in the oxidative addition of H2 to form two M-H bonds and, as such, the complexes might serve as a model for this process and for catalytic hydrogenation reactions by metal hydrides. Indeed, intermediate cases between and... 

Chemists refer to the bond in a molecule like sodium chloride as ionic , meaning that its electron pair resides entirely on chlorine. At the other extreme is the covalent bond in the hydrogen molecule, where the electron pair is shared equally between the two hydrogens. Intermediate cases, such as the bond in hydrogen fluoride which is clearly polarized toward fluorine, are generally referred to as polar covalent bonds (rather than partially ionic bonds). Are these situations really all different or do they instead represent different degrees of the same thing ... 

The reaction of benzoxazine in die presence of 2,6-xylenol does not occur until 135 C, presumably because die hydrogen-bonded intermediate depicted for the 2,4-xylenol reaction (Fig. 7.19) cannot occur. All three types of linkages are obtained in diis case. Para-para methylene-linked 2,6-xylenol dimers, obtained from the reaction of 2,6-xylenol with formaldehyde, formed in the decomposition of the benzoxazine (or with other by-products of that process) dominate. Possible side products from benzoxazine decomposition include formaldehyde and CH2=NH, either of which may provide the source of methylene linkages. Hie amount of ortho-para linkages formed by reaction of 2,6-xylenol with benzoxazine is low. Ortho-ortho methylene-linked products presumably form by a decomposition pathway from benzoxazine (as in Fig. 7.18). 

In some cases an alternative sequence involving addition of hydrogen at rhodium prior to complexation of the alkene may operate.11 The phosphine ligands serve both to provide a stable soluble complex and to adjust the reactivity at the metal center. The a-bonded intermediates have been observed for Wilkinson s catalyst12 and for several other related catalysts.13 For example, a partially hydrogenated structure has been isolated from methyl a-acetamidocinnamate.14... 

If the situation for Q and T is disappointing owing to their connections with C only, the position of the term R is intermediate between the two extremes (i) the R electrons belong purely to C (ii) the R electrons are shared equally between C and A. The former extreme is intuitively acceptable only if the C—A bonds are purely ionic, in which case the MO term anti-bonding loses much of its original significance. In (ii) and intermediate cases, the only excuse for excluding R from Eqn. 11.19 would be explicitly to assume Eqn. 11.13. 

The intermediate case is that the antibonding level of the molecule broadens across the Fermi level, as shown in Fig. A.13. It becomes partially filled, and consequently the intramolecular bond is weakened. The relevance for catalysis is that this type of chemisorption weakens (or activates in catalytic language) the... 

Intermediate cases in which the antibonding chemisorption orbital is broadened across the Fermi level can also arise (Fig. A. 14b). In such cases the antibonding orbital is only partially filled and the atom A will be chemisorbed, though with a weaker chemisorption bond than in Fig. A. 14a. 

In this chapter we illustrated how the CASSCF/CASPT2 method can be used to explore the nature of such chemical bonds. Classic cases are the Re-Re multiple bond in Re2Cl, and the Cr-Cr bond ranging from the quadruply bonded Cr(II)-Cr(II) moiety to the formal hextuple bond between two neutral chromium atoms. The bonding between the 3d5 electrons is weak and should be considered as an intermediate between two pairs of antiferromagnetically... 

Figures la or Id correspond to so-called weak bonding. Which of these two cases (a or d) actually takes place depends on the nature of the particle C and the lattice. Figures lb and Ic depict the strong acceptor bond. These two figures represent two limiting cases, the first of which (Fig. lb) corresponds to a purely homopolar and the second (Fig. Ic) to a purely ionic bond. In reality, as a rule, we deal with cases intermediate between Figs, lb and Ic. Figures le and If depict a strong donor bond. These figures also correspond to two limiting cases. In reality, we deal, as a rule, with intermediate cases.

We are currently trying to answer specifically the question of whether ir-bonded complexes do occur in certain cases where insertion reactions are observed. I think they do because I believe that the same factors which favor stabilization of this type of transition state will also tend to favor formation of 7r-bonded olefin complexes, which are only slightly removed from this. At the moment Bern Tinker is examining the insertion of olefins in mercuric complexes to see whether there is any indication of 7r-bonded intermediates. In his paper, Dr. Heck referred to some unpublished work relevant to this theme. I would certainly be interested in anything more he can tell us about that. 

Among the catalysts used are Lewis acids991 and phosphine-nickel complexes.992 Certain of the reverse cyclobutane ring openings can also be catalytically induced (8-40). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the -it or a bonds of the substrate.993 In such a case the reaction would of course be a concerted 2S + 2S process. However, the available evidence is more consistent with nonconcerted mechanisms involving metal-carbon a-bonded intermediates, at least in most cases.994 For example, such an intermediate was isolated in the dimerization of nor-bornadiene, catalyzed by iridium complexes.995... 

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