14-electron three-coordinated

The mechanisms of the reductive eliminations in Scheme 5 were studied [49,83], and potential pathways for these reactions are shown in Scheme 6. The reductive eliminations from the monomeric diarylamido aryl complex 20 illustrate two important points in the elimination reactions. First, these reactions were first order, demonstrating that the actual C-N bond formation occurred from a monomeric complex. Second, the observed rate constant for the elimination reaction contained two terms (Eq. (49)). One of these terms was inverse first order in PPh3 concentration, and the other was zero order in PPh3. These results were consistent with two competing mechanisms, Path B and Path C in Scheme 6, occurring simultaneously. One of these mechanisms involves initial, reversible phosphine dissociation followed by C-N bond formation in the resulting 14-electron, three-coordinate intermediate. The second mechanism involves reductive elimination from a 16-electron four-coordinate intermediate, presumably after trans-to-cis isomerization. 

Complexes of zero-valent palladium (d ) are tetrahedral if the coordination number is four, trigonal if the coordination number is three, and hnear if the coordination number is two. For Pd (d ), the geometry is square planar, unlike nF, which is normally octahedral, but like PF. With a very bulky ligand, 14-electron three-coordinate T-shaped species can be stabilized and structurally characterized. For example, the complex Pd(Ar)l[P(/-Bu)3], where Ar = 2,4-xylyl, can be isolated and the aryl group is opposite the open coordination site. For the few Pd (d ) complexes that have been structurally characterized, octahedral geometry is found. 

Two mechanisms are possible. Dissociation of PR3 from MC1(C0)(PR3)2 (M = Rh, Ir) would result in a 14-electron three-coordinate intermediate which could then react rapidly with PR 3, or dissociation could occur from a dimeric carbonyl or halide bridged intermediate (as proposed for the halide and carbonyl exchanges). Hard evidence for neither mechanism was presented. 

Although the oxidative addition to form the arylpalladium-halide complex usually controls the rate of the C-N coupling process, the reactions of the arylpalladium-amido complexes tend to control the scope and yield. Studies on the reductive elimination of arylamines were presented in Chapter 8. This reaction may occur from a 14-electron, three-coordinate palladium species or a 16-electron, four-coordinate palladium species. If the palladium is ligated by a monodentate dative ligand, then the reaction occurs from the three-coordinate complex, but if the palladium is ligated by a bidentate dative ligand, then the reaction occurs from a four-coordinate complex. ... 

Another remarkable exception to the eighteen electron rule is found among the d transition-metal ions, such as Ni(ll), Pd(II), Pt(ll), Rh(l), lr(l), and Au(ni), which often appear as four-coordinate square planar complexes with only 16 valence electrons. These are said to comply with the sixteen electron rule. Finally, d ions such as Cu(l), Ag(l), and Au(l) can also form sixteen electron three-coordinate complexes, or two-coordinate linear complexes that obey the fourteen electron rule. 

A related three-coordinate dicarbonyl-carbene complex can be obtained by reaction of Ni(CO)4 with the carbodi-phosphorane double ylide Ph3=C=PPh3. The reaction of Ni(GO)4 with Ph3=C=PPh3 in toluene gives the tricarbonyl complex Ni G(PPh3)2 (CO)3, but in THF solvent the deep red 16-electron three-coordinate complex Ni G(PPh3)2 (GO)2 was obtained. Both the three- and four-coordinate compounds Ni G(PPh3)2 (GO) , n = Z, 3) were structurally characterized by X-ray diffraction. ... 

Many functions, such as electron density, spin density, or the electrostatic potential of a molecule, have three coordinate dimensions and one data dimension. These functions are often plotted as the surface associated with a particular data value, called an isosurface plot (Figure 13.5). This is the three-dimensional analog of a contour plot. 

The discussion of the main group 3-5 and 3-6 compounds in the previous sections was limited to examples in which the group 3 element E is three-coordinate, so that an empty p-orbital on E is available for overlap with a lone pair on the group 5 or 6 atom. For the same reason, the discussion here will focus on those compounds with three-coordination at gallium, indium, or thallium. In the case of the transition metal derivatives, it is transition metal -electrons that are available to overlap with the empty p-orbital on E to form the potential ir-bond, as illustrated in Fig. 26. 

L2], could be formed in the presence of excess 1-methylimidazole but on recrystallization one ligand was lost to give the three-coordinate showing the balance between electronic and steric factors.304... 

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