18-electron systems, and

These six-co-ordinate compounds are undoubtedly 18-electron systems, and probably possess bent, reactive ArNj moieties. For example, reaction with... 

The PMeBu2 and PPr3 complexes react to attain an 18 electron system in various ways... 

The scientific interest in porphyrin ligands (Fig. 5) derives in part from their ability to accommodate a large series of different elements, often in various oxidation states. On the other hand porphyrins are planar molecules with a delocalized 18 Ti-electron system and a diatropic ring current [25], which makes them interesting for the design of new materials with applications in photochemistry [25-27]. 

Other recently developed techniques, such as immunoelectron microscopy (Chapter 18), electron systems imaging, and X-ray microanalysis (Chapter 19), should become routine practice in most EM laboratories and, thus, mainstays rather than ancillary EM methods. 

The configuration of rhenium(I) requires ligand systems which are able to accept electron density from the electron-rich metal center. Thus, frequently phosphines, nitrogen heterocycles, carbonyls, or isocyanides are encountered. Most of the octahedral products possess a high thermodynamic stability and kinetic inertness as is expected for 18-electron systems. 

Tetrapyrroles contain an extended jt-conjugated system which is responsible for their use in a wide range of applications ranging from technical (pigments, catalysts, photoconductors) to medicinal (photodynamic therapy) uses. The electronic absorption spectra are governed by the aromatic 18 tt-electron system and typically consist of two main bands. In phthalocyanines the Q band around 660-680 nm is the most intense one accompanied by a weaker Soret band near 340 nm . In porphyrins the situation is reversed with an... 

Because of their reversible electrochemical properties, ferrocene [biscyclopentadie-nyl-iron(II), FeCp2 and cobaltocenium [biscyclopentadienyl-cobalt(III), CoC p2 1 I are the most common electroactive units used to functionalize dendrimers. Both metallocene residues are stable, 18-electron systems, which differ on the charge of their most accessible oxidation states zero for ferrocene and + 1 for cobaltocenium. Ferrocene undergoes electrochemically reversible one-electron oxidation to the positively charged ferrocenium form, whereas cobaltocenium exhibits electrochemically reversible one-electron reduction to produce the neutral cobaltocene. Both electrochemical processes take place at accessible potentials in ferrocene- and cobaltocenium-containing compounds. 

Recently, clusters (18) with one capping oxygen atom and one capping ethylidyne moiety have also been prepared.237 The +1 cluster is prepared by refluxing W(CO)6 with a mixture of acetic add and its anhydride in MeCN. A one electron oxidation of the +1 ion leads to the +2 ion, whose crystal structure analysis reveals a W—W distance of 2.81 A. This distance is significantly longer than the one observed for (15), in agreement with the fact that the +2 ion is a five electron system and therefore the W—W bond order is less than unity. 

In contrast to the 18-electron systems, the 17-electron V(CO)6 is substitutionally labile. It is one of the few 17-electron systems that can be studied under non-transient conditions but there is considerable evidence that the conclusions drawn from the study of this system can be applied to transient 17-electron species such as Mn(CO)5. Data from a published study of CO exchange with V(CO)6 are absent but there is much anecdotal reporting. For example, a t/2 value of 7 h for the reaction in heptane at 10 °C under 2 atm of CO is quoted.97 The substitution reactions of V(CO)6 are associatively activated (k2 for the reaction between V(CO)6 and PPh3 in hexane at 25 °C =... 

Diazaborolines, isoelectronic with C5H5-, have been obtained by dehydrogenation of diazaborolidines (79) and from a,/3-diimines and halo-genoboranes (80). From UB- and 14N-NMR, UV, and PE studies, it was concluded that compounds of type 13 possess a 7r -electron system and therefore should be able to complex the Cr(CO)3 fragment. This was proved by the formation (91%) of the yellow complex 64 (80a) (air-stable decomp, above 170°C). As observed in borazine and diazadiborine complexes, only a small upfield shift (13, 8 = 26.2 64, 8 = 18.3 ppm) occurs in the B-NMR spectrum. Its constitution has been derived from lH and 13C-NMR, IR [jmass spectra data. (See Scheme 5.)... 

Iron-Pentacarbonyl. The FeVIII(CO)5 molecule is equally fundamental to organometallic chemistry and electrochemistry, and, like Feu(Cp)2, is a diamagnetic 18-electron system. It exhibits (a) an irreversible two-election oxidation and (b) an irreversible two-electron reduction (Figure 13.1e). In each case Fe(CO)5 has a synergistic effect on (1) the reduction of residual H20 and (2) the oxidation of solvent molecules ... 

Pcs are one class of such compounds with their delocalized two-dimensional 18 7t-electron systems and exceptional stabilities, which make them suitable candidates for NLO applications. From this point of view, Pcs, subphthalocyanines, and related compounds have been investigated extensively in recent years [14]. Besides the excellent 18 Tt-electron systems, the substituents at the benzene rings, coordinative metal, and axial coordination ligands of the Pcs affect the degree of the NLO properties significantly. However, correlation between these factors has not been fully established yet. 

It has been suggested that virtually all samples of MgO (and probably silicates as well) contain small amounts of HjO and CO2 (Freund, 1981). These impurities lead to formation of O species. The species CO4 " has also been postulated to occur on the surface of MgO exposed to COj. This species has recently been studied by ab initio 8CF Hartree-Fock-Roothaan MO calculations, both in its anion form and as the protonated cluster C(0H)4 (Gupta et al., 1981). Its calculated equilibrium bond distance is intermediate between those observed for B emd N in tetrahedral coordination with oxygen, and there seems to be no intrinsic source of instability. Thus, such a species seems stable. However, the calculations indicate a charge on C in 04" " very similar to that in COj, arguing for the formulation O 04 as first approximation to the electronic structure, rather than the C°(04 ) formulation suggested by Freund (1981). Perhaps such a species is better formulated as a chemisorption complex of a anion (a bent, 18-valence-electron system) and an O. . . 0 ... 

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