5-Coordinate intermediates, structures

Schiff base crown ether ligands, 39 118-123 5-Coordinate intermediates, structures, 34 230-... 

Figure 6. Depiction of the equilibrium between two possible six-coordinate intermediate structures generated via ligand dissocciation from a seven-coordinate intermediate Mn(ll) complex.

There may be more than one TS connecting two minima. As many of the interpolation methods start off by assuming a linear reaction coordinate between the reactant and product, the user needs to guide the initial search (for example by adding different intermediate structures) to find more than one TS. 

A simple example serves to illnstrate the similarities between a reaction mechanism with a conventional intermediate and a reaction mechanism with a conical intersection. Consider Scheme 9.2 for the photochemical di-tt-methane rearrangement. Chemical intnition snggests two possible key intermediate structures, II and III. Computations conhrm that, for the singlet photochemical di-Jt-methane rearrangement, structure III is a conical intersection that divides the excited-state branch of the reaction coordinate from the ground state branch. In contrast, structure II is a conventional biradical intermediate for the triplet reaction. 

Intermediate between the extended four-coordinate connectivities that dominate the low- T solid phase and the two-coordinate ring/chain connectivities that dominate at higher T are certain //jree-coordinate polyhedral structures that retain a degree of cooperative proton ordering. Two examples of such trigonally coordinated buckyball clusters, a 24-mer and a 28-mer, are shown in Fig. 5.31. The... 

What specific properties of these complexes have allowed isolation of five-coordinate Pt(IV), in the form of the trimethyl complex and the dihy-dridosilyl complexes These two types of complexes are significantly different, and their stability is apparently due to different factors. Comparing the trimethyl complex in Scheme 21(A) with the related but six-coordinate complexes of a similarly bulky oc-diimine ligand (98), shown in Scheme 23, is instructive. In Scheme 23A, triflate is clearly coordinated, exhibiting an O-Pt distance of 2.276(3) A (98), which is typical for Pt-coordinated triflate (108). This triflate complex A in Scheme 23 was obtained from dry tetrahydrofuran. The aqua complex cation B, also structurally characterized, was obtained from acetone containing trace water. An equilibrium between coordinated triflate and coordinated water, very likely via a common five-coordinate intermediate, was indicated by NMR spectroscopy (98). 

With specially designed reactants, the determination of the product structure can be very informative. Mercury(II)-catalyzed aquations of Co(III) complexes are believed to proceed via a 5-coordinated intermediate (Sec. 4.3.2). The shape of this intermediate is of interest. The Hg +-catalyzed aquation of Co(NH3)4 (NDj)X + in which the ND, and X groups are trans to one another gives substantially rrans-Co(NH3)4(ND3)(H20). This is excellent evidence for a square pyramidal intermediate in the reaction. A trigonal-bipyramidal intermediate would be expected to lead to substantial scrambling of the NDj and NHj groups (Fig. 2.1). The definite but very small amount (2.8 0.4%) of cis product recently reported using NHj instead of NDj attests to the sensitivity of current nmr machines. 

Perhaps the most controversial suggestion is that spontaneous isomerizations may proceed through a dissociative mechanism, invoking two three-coordinated intermediates (probably T-shaped). The labile intermediates may be cis-like (A) or trans-like (B) in structure. Applied to the monoorganoplatinum(II) complexes, PtL2XR, the scheme is ... 

Figure 20 Possible mechanism of formation of the compound Ba2YCu2 89M0 n07 0 with M in six-fold coordination, (a) Shifts of the atoms are indicated by the arrow, (b) Intermediate structure, (c) Resulting structure.

Ralph Pearson I would like to go back a couple of questioners and answer Dr. Halpern s question. I wrould certainly like to believe that the accelerating effect of the hydroxyl group is caused by ir-bonding s stabilizing a five-coordinated intermediate wdth a trigonal bipyrimidal structure. 

FIGURE 29. Reactant cluster, transition state, TS, and the IRC path study (right drawing) of the epoxidation of allyl alcohol with peroxyformic acid showing the movement of atoms from the transition state (dark, PI) toward the products (light, P3) with an intermediate structure, P2. The calculation was done at the MP2/6-31G(d) level. The reaction coordinate is in units of amu bohr, the relative energies are in kcal mol-1 and the distances are in A. Geometric parameters in parentheses are at the MP2/6-3 lG(d,p) (see text) level of theory... 

The observed decays for E = 0 (and at higher energies) give the rates at which the BPS is changing with time due to proton transfer. The fact that the initial tautomerization is on the femtosecond time scale, when the total vibrational energy is zero, indicates that the proton transfer motion is direct and does not involve the entire vibrational phase space of the pair. The implication is that the motion can be described as localized in the coordinate of N—H N. Furthermore, the two decay components indicate the presence of the intermediate structure, which reflects the two-step motion in the transfer. 

Rg. 6.18 Real five-coordinate molecular structures illustrating intermediates between TBP (iy on top left to SP (G ) on the bottom nght. Note that two different NKCN) polyhedm are found m the unit cel of the Cr(en)j salt. (From Muetterties E. L Guggeffcerger, L J. 

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