Coordination bond rupture

Variational RRKM theory is particularly important for imimolecular dissociation reactions, in which vibrational modes of the reactant molecule become translations and rotations in the products [22]. For CH —> CHg+H dissociation there are tlnee vibrational modes of this type, i.e. the C—H stretch which is the reaction coordinate and the two degenerate H—CH bends, which first transfomi from high-frequency to low-frequency vibrations and then hindered rotors as the H—C bond ruptures. These latter two degrees of freedom are called transitional modes [24,25]. C2Hg 2CH3 dissociation has five transitional modes, i.e. two pairs of degenerate CH rocking/rotational motions and the CH torsion. 

Monomeric sulfur diimides have an extensive coordination chemistry, as might be anticipated from the availability of three potential donor sites and two re-bonds.131 Under mild conditions with suitable coordinatively unsaturated metal complexes, sulfur diimides may coordinate without rupture of the -N=S=N- unit. Four modes of coordination have been identified or invoked as intermediates in fluxional processes (Scheme 8). 

There are basically two types of intramolecular rearrangements which are not easy to distiguish experimentally. In one type, intramolecular twisting occurs without any metal-donor atom bond rupture. In the other, rupture of one metal-ligand bond of the chelate occurs to give a five-coordinated intermediate. 

The basic classification of nucleophilic substitutions is founded on the consideration that when a new metal complex is formed through the breaking of a coordination bond with the first ligand (or water) and the formation of a new coordination bond with the second ligand, the rupture and formation of the two bonds can occur to a greater or lesser extent in a synchronons manner. When the mpture and the formation of the bonds occur in a synchronous way, the mechanism is called substitution nucleophilic bimolecular (in symbols Sn2). On the other extreme, when the rupture of the first bond precedes the formation of the new one, the mechanism is called substitution nucleophilic unimolecular (in symbols SnI). Mechanisms Sn2 and SnI are only limiting cases, and an entire range of intermediate situations exists. 

Fig. 13. Coordination fluctuations of Nb5+ in the decaniobate ion during a molecular dynamics calculation of the decaniobate ion in continuum solvent. Oxygen atoms are red, and niobium atoms are green. The dynamics are characterized by frequent Nb— i.3-0 bond ruptures, as shown in (b), followed by instantaneous tetrahedral coordination for the Nb3, shown in (c). See Figure 4 for description of notation.

The powerful role of the exitonic migration was proved on the basis of the luminescence and photosensitivity investigations [270]. The preliminary ultraviolet illumination of PAC increases the photosensitivity and decreases the luminescence. The experimental data are given in Fig. 40. One can see the redistribution of the maxima intensity in the spectra without changing their positions. Apparently ultra violet illumination promotes the photolysis of the weak coordination bonds. This leads to the changing of the polymer homolog content. Stimulated exciton dissociation on the ruptured bonds results in an increase in the photosensitivity and a luminescence decrease. The experiments carried out at 77 K show that in the luminescence spectrum of irradiated frozen PAC a new maximum appears with a position close to the phosphorescence maximum of diphenylbutadiene. So the rupture of weak coordination bonds under ultraviolet irradiation was proved. 

Resonances with pure excitation of the CO stretching mode (0, U2, 0) (an example with Vi - 8 is shown in Fig. 5) have the smallest rate and therefore the longest lifetime energy transfer from r to R is rather inefficient, and therefore the system needs a long time before enough eneigy is accumulated in the dissociation coordinate to permit dissociation. On the other extreme, direct excitation in R allows a rather rapid bond rupture, and therefore the resonances (t>i, 0, 0) have the shortest lifetimes. Excitation of the bending mode (0,0, U3) leads to lifetimes that are between C-O excitation as the lower limit and H-C excitation as the upper extreme. This mode specificity is further elucidated in Fig. 8, where we show the widths for several... 

Despite considerable biochemical interest in the naturally occurring triamine 3,4-tri, spermidine (31), surprisingly few coordination compounds have been reported. The crystal structure of Cu(3,4-tri)Br2 (five-coordinate, square pyramidal) has been determined678 and mer-Co(3,4-tri)X3 (X = NOz, Cl) have been prepared.679 The triaqua complex resulting from the latter is unstable with respect to spontaneous thermal reduction in acid solution (presumably via Co—N bond rupture) to give Co". 

Rearrangements of six-coordinate tris(diketonate) metal complexes have been the subject of numerous intense and careful investigations and the great body of literature has been incisively reviewed.77,283 A principal conclusion is that in no case has a unique rearrangement been unambiguously established and indeed, one may not exist. The difficulty arises from the consideration that the various five-coordinate transition states (or intermediates) which result from bond rupture processes can have quite similar energies. Thus, combinations of mechanisms can obtain which lead to extremely complicated DNMR spectra or isomerization kinetics. 

A heterolytic rupture will, in the case of an ordinary covalent bond, lead to formation of a cation and an anion in the case of a coordinate bond, the ligand simply departs along with the electron pair it contributed to form the bond. A dissociative mechanism exhibits first-order kinetics its rate is independent of the concentration of the incoming group Z. The intermediate EX in the overall reaction ... 

A qualitative structural model has been proposed by these authors. Before the photolysis, the 6-coordinated Fe2+ ion is located in the plane of the porphyrin ring of the heme unit. Rupture of the coordination bond... 

Mechanisms for such reactions have been viewed by the effective coordination number of the transition state. Bond rupture processes proceed via five-coordinate transition states for which idealized square-pyramidal (SP) and trigonal-bipyramidal (TBP) geometries with the dangling ligand axial or equatorial have been considered. Twist mechanisms are considered to proceed via six-coordinate transition states with idealized trigonal prismatic (TP) geometry. Twists of the chelate rings about the real or pseudo C3 axis in the case of the cis or trans isomer, respectively, and about the... 

Since the moment of inertia ratio will be nearly unity, the major source of the isotope effect at a given energy is the activated complex ratio. A pure intramolecular statistical secondary effect can scarcely arise, since by definition no net differential effect on the competing reaction coordinates arises due to isotopic substitution. However, in practice, small mechanistic effects mentioned earlier can exist for isotopic substitution which preferentially affects one of the competing bond-rupture sites. 

There are few reports of reactions between alkynes and trinuclear clusters of metals other than iron, ruthenium, or osmium. Some rhodium, platinum, and mixed-metal clusters undergo metal-metal bond rupture in reactions with alkynes (54-56), while in other cases the alkyne coordinates to the trinuclear unit without causing any major changes in framework geometry (56-59), as illustrated in Eq. (3). 

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