A-frame species

Isolation of the neutral, face-to-face dimers with dmpm, but the ionic, A-frame species with dppm, may be rationalized in terms of steric congestion. Ionization of the halide diminishes the crowding in the molecule, and this becomes more important for the laiger dppm ligand (90). 

The neutral face-to-face complexes [Ir2X2(CO)2(dppm)2] (X = Cl, I), have been found to be in equilibrium with their related cationic A-frame species through the dissociation of one halide ligand.[8, 11] Not surprisingly, the addition of dihydrogen to both types of compounds has been found to involve common intermediates, although the final reaction products are different.[8,10]... 

Lithium 1,2,4-triazolate with [Rh2( j,-Ph2PCH2PPh2)(CO)2( j.-Cl)]PFj. gives the A-framed complex 177 (L=L = CO) (86IC4597). With one equivalent of terf-butyl isocyanide, substitution of one carbon monoxide ligand takes place to yield 177 (L = CO, L = r-BuNC), whereas two equivalents of rerr-butyl isocyanide lead to the product of complete substitution, 177 (L = L = r-BuNC). The starting complex (L = L = CO) oxidatively adds molecular iodine to give the rhodium(II)-rhodium(II) cationic species 178. 

As we will discuss, such a symmetric profile is typical of an electron removal which does not lead to important structural changes. In fact, the 17-electron ferrocenium ion, [Fe(C5H5)2] +, generated upon oxidation, is a stable species which substantially maintains the original molecular frame (but for the fact that, because of the electron removal, the iron-carbon bonds are slightly weakened and hence elongated by about 0.04 A with respect to the neutral parent see Chapter 4, Section 1.1). 

There are also many ways of expressing the velocity of a chemical species present in a flow system. We do not concern ourselves here with the instantaneous velocity of the individual molecules of a species, but rather with the average macroscopic velocities with which the species travel. These may be measured from a stationary coordinate system, but for flow systems the velocities of individual species are frequently measured from a coordinate frame moving with (a) the mass-average velocity of the stream, (b) the molar average velocity of the stream, or (c) the velocity of one particular component. The mass- and the molar-average velocity are defined in Table II and the notation for the various velocities of an individual species is given. 

The colossal edifice of evidence has created a very strong cause and effect link between 7r-delocalization and geometry, with a lot of emphasis on the preference of the 7r-electron energy. A consensus seemed to have been reached that the symmetric geometries (with uniform C—C bond lengths) in species such as benzene or allyl result from the inherent tendency of n-electrons to undergo bond delocalization in the absence of constraints by the a-frame.1319 The extent of the consensus may be witnessed from papers94-95 on barriers of identity Sn2 reactions such as Cl- + RC1. The authors reason that since the (Cl—R—Cl) -transition state is isoelectronic to allyl anion, which... 

Figure 4. The a- and jr-curves for a bond-delocalized species. The d and d/, on the bond alternating coordinate represent small distortions (smaller than the extremes R and L) permitted by the a-frame. The bottom two diagrams show the total energy curves for archetypal species.

Antiaromatic molecules have, as a rule, very low-lying triplet states, ca. 10 kcal/mol or less above the ground state.89 187 Furthermore, since the a-frame limits the distortivity of the antiaromatic molecule, the triplet excitation is constrained to remain small. Consequently, the instability of antiaromatic species is a kinetic instability associated with availability of low-lying triplet states. A related argument has been used recently by Zilberg and Haas.188... 

It is necessary to state that the flux, and, consequently, the diffusion coefficient, as discussed later, have to be chosen relative to a frame of reference, since the diffusion flux,./, gives the number of species crossing a concrete unit area in the medium per unit of time [13]. 

If concentration gradients are assumed negligible except in the direction normal to the material interface,the mass conservation equation for species i in a frame 7, attached to the lamellae, that translates and rotates with the flow is ... 

The phosphine-bridged Pd(I) and Pt(I) dimers, M2(dppm)2X2 (dppm = Ph2PCH2PPh2), contain unusually reactive metal-metal bonds that are susceptible to insertion by a variety of species Y to form A-frame bridged structures. 

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