Bridging moieties

Various dinickel(II) complexes related to (724d) but with a phenolate bridging moiety, type (724e), have been investigated. To illustrate the great variety, (729) and (730) are depicted as examples.1883 1884... 

Rhodium(II) forms a dimeric complex with a lantern structure composed of four bridging hgands and two axial binding sites. Traditionally rhodium catalysts faU into three main categories the carboxylates, the perfluorinated carboxylates, and the carboxamides. Of these, the two main bridging frameworks are the carboxylate 10 and carboxamide 11 structures. Despite the similarity in the bridging moiety, the reactivity of the perfluorinated carboxylates is demonstrably different from that of the alkyl or even aryl carboxylates. Sohd-phase crystal structures usually have the axial positions of the catalyst occupied by an electron donor, such as an alcohol, ether, amine, or sulfoxide. By far the most widely used rhodium] 11) catalyst is rhodium(II) acetate [Rh2(OAc)4], but almost every variety of rhodium] 11) catalyst is commercially available. 

Asymmetric isomerization of 4-terr-butyl-l -vinylcyclohexane (6) catalyzed by bis(indenyl)-titanium complex (8) bearing a chiral bridging moiety afforded (S)-4-fcrr-buty 1-1 -ethy lidenecy-clohexane (7) with up to 80% ee (Scheme 3A.3).3,4... 

CEI)313 and simulated CEI studies314 showed, however, that the structure is not planar, but the protons trans-bent with respect to the plane of the bridge moiety. 

Figure 11.5 Diagrammatic explanation of the coulombic and electron exchange energy transfer mechanisms (A and B are chromophore components and L is a bridging moiety or ligand).

Tab. 2 Racemization barriers in overcrowded alkenes 23 with different bridging moieties X and Y.

Table 2 summarizes the racemization barriers in unsubstituted chiral alkenes 23 with different bridging moieties in their upper and lower halves. As is evident from these data, the tetrahydrophenanthrene-type upper part is large enough to prevent fast racemization by movement of the aromatic moieties of upper and lower halves through the mean plane of the molecule. On the other hand, there is enough conformational flexibility in the molecules to prevent excessive distortion of the central olefmic bond (leading to ground state destabilization), which would lower the racemization barrier. 

Mononuclearity in Ln(II) CH-alkoxides has not yet been proven by X-ray crystallography [28,55,56]. The dimeric heteroleptic complex (Yb[N(SiMe3)2] ( -tritox) 2 established the alkoxide ligand as the bridging moiety [28]. Mono-europium(II) complexes with tri-fert-butoxygermanate-, -Stannate and -plum-bate ligands were reported and Eu [Sn(OtBu)3]2 was characterized by X-ray structure analysis (Table 2) [55b]... 

Now, according to Eq. (257) the effective Hamiltonian describing the two H-bond bridge moieties, when both the fast modes are in their ground states, is... 

Hamiltonians (P.26) are those of the undriven quantum harmonic oscillator describing the H-bond bridge moieties a and b. Hamiltonian (P.27) is that of the driven quantum harmonic oscillators describing the a H-bond bridge moiety. Finally, Hamiltonians (P.29) are dealing with the coupling of the H-bond bridge with the thermal bath, whereas Hamiltonian (P.28) is that of the thermal bath. 

Coordinates and conjugate momentum of the a and b H-bond bridge moieties. Coordinates and conjugate momentum of the symmetrized g and u H-bond bridge moieties. 

Many /x-oxo dimers are not stable under reducing conditions owing to instability of the n-oxo bridge moiety in either the mixed-valence or diferrous states. For example, [Fe(HBpz3)]20(0Ac)2 (HBpz3 is hydro-tris(pyrazolyl)borate) shows an electrochemical irreversible reduction wave even at very high scan rates (5 V/s) (13). The fact that the ascorbic acid does not show the same reactivity as 2-mercaptoethanol may be a consequence of differences in the redox potentials or the ability of the reductant to displace ligands and coordinate to the metal center. 

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