Condensed Imidazoles: 5-5 Ring Systems (The Chemistry of by P. N. Preston
By P. N. Preston
This quantity includes a survey of fifty one ring platforms within which an imidazole ring is fused to an extra 5-membered ring approach. Ring structures incorporated are these showing within the Chemical Abstracts Index of Ring structures lower than the 2-ring 5-5 quotation and together with a C3N2 fragment. the cloth is for this reason subdivided into chapters in line with the quantity and kinds of straight atoms within the extra five-membered ring. The synthesis, physiochemical houses and reactions of compounds in each one ring method are coated; reactions are prepared on a mechanistic foundation and the survey contains compounds which are either partly and entirely saturated.
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Extra resources for Condensed Imidazoles: 5-5 Ring Systems (The Chemistry of Heterocyclic Compounds, Volume 46)
165 31 B,4. ( + I l ) OXIDATION STATE COMPLEXES magnetic with two unpaired electrons. An alternative for very bulky ligands or for covalent ligands which can π bond to the metal is for a tetragonal dis tortion to take place. This implies the removalf of two trans ligands; as a consequence of the lowering of the symmetry to D the degeneracy of the d orbitals is further lifted. Two (d 2_ and d ) are raised in energy. The remain ing three (d and d which are degenerate, and d 2) are lowered. The usual order quoted for Ni(II) is d i_ > d > d i > d = d , but for Pd(II) and Pt(II) the d 2 level, as might be expected, has been shown to be lowest from the electronic spectra and the levels are now d 2_ 2 > d > d = d > d 2 .
Tolyl)Cl + py -> /ra«s-(Et P) M(o-tolyl)py + Cl~ + 3 2 3 2 the relative rates are 5 χ 10 (Ni); 1 χ 10 (Pd); and 1 (Pt). These authors have interpreted this result in terms of the greater ease with which Ni(II) and Pd(II) can achieve 5-coordination by comparison with Pt(II). Smith and 6 t See Glossary. 5 193 37 B,4,d. ( + I l ) OXIDATION STATE COMPLEXES: KINETICS Sawyer have estimated that Pd-O exchange must be at least 10 as fast as Pt-O exchange in [(HOOCCH ) NCH COO] M. 194 3 2 2 2 2 e. Trans Effect, Trans Influence, and Bond Lengths The occurrence of a trans effect was first noticed by Chernyaev and co workers in their syntheses of square planar Pt(II) complexes.
The M(II) Oxidation State For nickel and palladium this represents by far the most important state in most of their normal chemistry and for platinum it is at least as important as the (IV) state. a. Electronic Structures for the d State s In the absence of ligands which make special demands on the metal, Ni(II) is usually paramagnetic and octahedrally coordinated, whereas Pd(II) and Pt(II) are diamagnetic and square planar. The (II) oxidation state has the d configura tion. For a field of six ligands of octahedral symmetry, the degeneracy of the d orbitals in the free ion (in the valence state) is lifted and two orbitals (d 2_ 2, d 2 ) are raised in energy by ^Δ , the remaining three becoming stabilized by an equivalent amount, f Δ , where Δ is the crystal field splitting energy for an octahedral field (see Fig.