Chelate cycle

KPFfi differently, yielding 225 and 226, respectively. In turn, the same combination of the reactants but with the ligrmd 227 gives species 228 with the eight-membered chelate cycle. 

Special interest has recently been given to the metal 2-alkoxyethoxides, M(OC2H4OR)Il (R= Me, Et) — derivatives of the monoethers of ethyleneglycol. The presence of the second ether function in the molecule permits formation of chelate cycles ... 

The products of partial substitution for the OR-groups, i.e., alkoxocarbox-ylates, P-diketonates M(OR)mX , where X= RCOO, acac, thd, etc.—the mixed-ligand complexes—usually are more soluble and stable than M(OR) , as their molecules contain the chelate cycles. Mehrotra et al. [223] have described a great number of such products (usually liquid or amorphous), in which individuality appeared sometimes questionable. During recent years, however, in connection with the application of these compounds to the synthesis of oxides, interest in them has increased, and a number of them have been isolated and structurally characterized (Chapter 7). Unfortunately, length restriction did not permit us to present a complete review of their chemistry in this book. 

In general, pentacoordinate silicon complexes have a five-membered chelate cycle. Few exceptions are known with larger (six-membered, 102)" and smaller (four-membered 10397a, 10497b) chelate cycles. 

The compound 614 has an interesting alterdentate ligand system (because of a possibility to form chelate cycles on different N atoms), which forms a chelate 615 having five-member metal-cycle [62] ... 

It should be noted that the entropy component of the polychelate effect is largely controlled by the same contributions as for LMWM, i.e., translation, rotation, symmetry, isomerism, oscillation, internal rotation and solvation contributions [41]. Besides both the chelate and polychelate effects depend greatly on the size of the chelate cycle. In this case the PCMU obey the known Chugaev rule by... 

In the rearrangement reaction leading to 10a and 11a the nitrogen atom which is nearest and most accessible to the chloromethyl group displaces the chlorine atom in an internal SN2 type substitution, followed by migration of the displaced chloride to silicon. This rearrangement leads to the two different products, with five- and six-membered chelate cycles, 10a and 11a. While five-membered chelate cycles as in 10a are common in hypercoordinate silicon chemistry, the formation of six-membered chelates (11a) is unique.33,34... 

The two pairs of protonated units condense with amino groups to form six-membered chelate cycles (the so-called straps ) cross-linking two tame fragments to produce two tetradentate moieties. The two remaining primary amine groups, being in the trans position, then condense with propanal to form bridges ( cross straps ) between straps. 

In the case of amidine- and amide-functionalized cobalt(II) Ne- and NsSs-sarcophaginates, two of the three five-membered chelate cycles display the lei conformation, while the third one, which is on the same strand as the amide or amidine group, has the ob conformation... 

The average of the main structural parameters of chelate cycles. ... 

The ethoxy groups in boron-capped complexes with TP geometry are located between petals formed by three planar oximehydrazonate chelate fragments. In addition, the C-C bonds in chelate cycles are oriented parallel to the Cs symmetry axis of the clathrochelate molecule (i.e., ZeZ-conformation), and the substituents at these atoms induce no steric hindrances in the space between petals. ... 

The mechanism proposed for the formation of the sepulchrates accounts for the fact that, despite seven chiral centres and six nitrogen-metal bonds, the synthesis of [Co(sep)] + cation yields only one isomer. After the formation of the first heminal diamine and second imine centre, the chiral nitrogen centre must be oriented so that the methylene unit of the amine fragment and the proton are apical and equatorial, respectively, before intramolecular cycli-zation. In this case, six-membered ring closure takes place. The same apical orientation of the six-membered chelate cycle must be retained to complete the formation of a capping group. After the completed encapsulation, the methylene unit and the proton cannot... 

Reaction (4) on Ni is promoted by addition of alkali metal hydroxides in solution. We have found 426) that they form a chelate cycle with the fAreo-hydroxyls of the hexite... 

The chelate complex is shown in Figure 9. As the crystal is centrosymmetric, d and 1 forms of optically active Co(en)g are present in the structure in equal amounts. The" atom of cobalt is in an almost regular octahedron composed of nitrogen atoms. Each atom of chelate cycle is located in the tetrahedron two tops of which are occupied by the neighbouring atoms of the ethylenediamine ring and the two others by the hydrogen atoms. 

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