U-Complexes

The effect of the bond dipole associated with electron-withdrawing groups can also be expressed in terms of its interaction with the cationic u-complex. The atoms with the highest coefficients in the LUMO 3 are the most positive. The unfavorable interaction of the bond dipole will therefore be greatest at these positions. This effect operates with substituents such as carbonyl, cyano, and nitro groups. With ether and amino substituents, the unfavorable dipole interaction is overwhelmed by the stabilizing effect of the lone-pair electrons stabilizing 3. 

Two further unsaturated cages. Each of the systems discussed so far involves reaction of an electrophilic reagent with non-coordinated nucleophiles appended to metal-bound ligands. In contrast, in the following synthesis, cage formation occurs via an internal rearrangement of an Fe(u) complex of type (151) (Herron et al., 1982). Complexes of type (151) have already been discussed in Section 3.5. Treatment of these... 

Fig. 41.11 R u-complex and task-specific ionic liquid for the hydrogenation of acetophenone derivatives.

Se is concentrated in certain ore deposits, such as roll-front U complexes and Carlin-type gold deposits. Roll-front U deposits occur where U(Vl)-bearing groundwater migrates into reducing zones which precipitate both U(TV) and Se(0) or Se(-II). It may be possible to constrain Se sources and/or ore-forming processes using Se isotope variations. 

Mulliken s model of the transition state (Pickett et al., 1953 Muller et al., 1954) is related indirectly to the structure of u-complexes. It is based on the idea of hyperconjugation effects stabilizing the transition state, and, although not directly invoking reactivity indices, must be mentioned as perhaps the most satisfactory description given so far. 

The preceding section summarized basicity results obtained from the stability constants of 7r-complexes and EDA-complexes. These results can only reflect a qualitative gradation of the basicity. If one moves from these complexes to u-complexes, then exact values for the basicity of unsaturated compounds can be obtained by measuring the formation equilibria of the proton addition complexes in strongly acid solutions. The experimental methods and the calculations have been described in Sections III, A-C. 

Figure 1 shows that concentrations of U and associated elements such as Mo, La, Ce, Au, and Th tend to increase in fine fraction of soils over the uranium deposit. This is due to the fact that U complexes [U02] " are easily adsorbed by clays. Figure 2 shows that U and Mo in -120 mesh fraction of soils give good indication to uranium deposit at the depth of 300m. 

Two steps must be considered in the mechanism of homolytic acylation, in addition to the formation of the acyl radical. The first fits in with the generally accepted mechanism of homolytic aromatic substitution, that is, the addition of the acyl radical to the aromatic nucleus to give an adduct in which the unpaired electron is delocalized over the residual heteroaromatic system (u-complex 6). 

Note that there is a degree of arbitrariness associated with the complex representation of a real field F could just as easily have been written F - Re(F, where F = C exp(/to/) and the asterisk denotes the complex conjugate. Thus, there are two possible choices for the time-dependent factor in u complex representation of a time-harmonic field exp(/co/) and exp( — iat). It mukcs no difference which choice is made the quantities of physical interest arc ttlwuys real. But once a sign convention has been chosen it must be used conniMcntly in all analysis. We shall take the time-dependent factor to be exp(-/u>/) this is the convention found in standard books on optics (Born Mini Wolf, 1965) and electromagnetic theory (Stratton, 1941 Jackson, 1975) as... 

The U complex, having no net charge and already containing TBP solvent ligands, passes preferentially into the TBP solvent phase, leaving the Pu3+ and almost all the fission products such as Sr2+, I-, and Cs+ behind in the aqueous phase. 

General agreement on what to call these ions has not yet been reached. The term u complex is a holdover from (he time when much less was known about the structure of carbocations and it was thought they might be complexes of (he type discussed in Chapter 3. Other names have also been used. We will call them arenium ions, following the suggestion of Olah J. Am. Chem. Sac. 1971, 94, 808. 

Several four-coordinate selenium(U) complexes with bidentate ligands, all of which involve highly distorted square planar SeS4 or SeSe4 coordination spheres, and some tellurium analogues have been described.11... 

The nitrosyl complexes of nickel(II) are scarce and less studied than those of nickel(0) (see Section 50.2.5.2) even though they have been known for nearly a century. Selected examples of nitrosyl nickel(U) complexes are reported in Table 58. As early as 1891 it was reported by Berthelot1117 that Ni(O0)4 reacts with gaseous NO giving a blue compound which was later characterized as a pseudotetrahedral complex of nickel(U) having the formula [Ni(OH)3(NO)].1118 This compound is paramagnetic and is formed only if traces of water are present in the reactants. Using a methanolic solution of Ni(CO)4 a methoxo derivative is formed. 

The number of nickel(U) complexes with mono-, bi- and poly-dentate ligands containing tertiary phosphines as a donor group is very large and increases day by day while complexes with tertiary arsines are less numerous and those with stibines are rarer still. The number of nickel(II) complexes with mixed donor ligands containing N, O and S donor atoms besides P or As is also very large. 

Selected nickel(U) complexes with various monodentate phosphoryl esters are reported in Table 86. It is generally found that the NiL2+ and NiL2+ species are converted to the hydrated species NiL4(OH2)2+ on exposure to aerial moisture. 

Nickel(lll) deprotonated peptide complexes can be easily obtained in solution by chemical or electrochemical oxidation of the corresponding nickel(U) complexes. They are moderately stable in aqueous solutions and have been widely characterized by EPR, electron spectroscopy and cyclic voltammetry.3047,3058,3087,3088 Some selected examples of nickel(III) peptide complexes are reported in Table 115. 

ESR spectra obtained from magnetically dilute samples of the silver(U) complexes doped into the corresponding dimeric zinc complex could be attributed to both homo- (Ag,Ag) and hetero- (Ag,Zn) dinuclear species. In the Ag,Ag dimer the g values were almost equal to those for the Ag,Zn dimer however, the hyperfine coupling constants A were only about half. From this it was concluded that the structures of these two species were very similar.542... 

The first evidence for gold(II) was obtained from a study of the Fen-catalyzed exchange of chloride with [AuCL ]-. The first step was thought to involve formation of a labile gold(U) complex formed according to equation (58).457... 

Mixed halide-thiocyanate compounds Hg(SCN)X (X = Cl, Br, I) are formed from equimolar amounts of the pure components. They contain six-coordinated mercury(H) achieved by bridging X and SCN groups.234 The formation constants of the mixed thiocyanato complexes have been detected spectroscopically.233 Raman spectra of mixed halothiocyanatomercurate(U) complexes have been reported by Cooney and Hall.236 The structure of ammonium... 

These data are rationalized in terms of the Shannon-Swan rule and the low first ionization potential of the Cu(II) ion compared to the Zn(II) ion. This difference allows the Zn(U) complex to fragment by the route, OE - EE + OE , (i.e., a radical loss from the molecular ion). The fragmentation scheme of the Cu(II) complex follows the route, EE - EE + EE (i.e., a neutral, stable molecule loss from the molecular ion), which suggests an internal electron transfer and cleavage of the Cu-S bond in the molecular ion (612). 

LIGAND. Any atom, radical, ion, or molecule in u complex ipnly-ulomic group) which is hound lo the central aiorn. Thus, the ammonia molecules in Co(NH-,l(,, ". and ihe chlorine atoms in PtCIn) arc ligands. Ligands are also eomplcxing agents, as for example. EDTA. ammonia, etc. See also Chelates and Chelation. 

In eqs. (7a) and (7b) v) is a matrix of one column containing the components of v, and u is a matrix of one row, which is the transpose of w ), the matrix of one column containing the components of u, complex conjugated. In eq. (6), transposition is necessary to conform with the matrix representation of the scalar product so that the row x column law of matrix multiplication may be applied. Complex conjugation is necessary to ensure that the length of a vector v... 

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