Anionic ligands

When there are several types of ligands, anionic ligands are cited before the neutral ligands. 

As a consequence of the more electron withdrawing character of the boratabenzene ligand, anionic borabenzene complexes are greatly stabilized as compared to their Cp counterparts. The bis (boratabenzene) metal complexes 32 of V (62), Cr (62), Fe (below -10°C) (62), and Co (173) show fully reversible one-electron reductions at easily accessible cathodic potentials. 

Mixed-ligand anions [RC=CAuX] are rare in organogold chemistry. Selected examples have been obtained from reactions of [RC=CAuL oligomers with halides Q+X, from Q[Au(acac)2] salts with alkynes, followed by acids HX in the molar ratio 1 1 1, and from complexes (tht)AuCl (tht = tetrahydrothiophene) or [(Ph3P)2C]AuCl with an alkyne and a base (1 1 1). Known examples of products include salts Q+[HC=CAuCl/Br]-72 and [(Ph3P)2CH]+[PhC=CAuCl]. 52,53 Anions with mixed alkynyl/aryl substituents are present in... 

Once the optimal metal/ligand/anion combination has been determined, the choice of the metal complex which is actually put into the reaction solution will become of importance. Active hydrogenation catalysts are metal-ligand complexes which can either be prepared in situ simply by mixing a suitable precur-... 

Ligand Anion Support Substrate PHj Rh ratio [MPa] Cycle number TOF [h-1] ee [%i Reference... 

The adsorption of ligands (anions and weak acids) on metal oxide (and silicate) surfaces can also be compared with complex formation reactions in solution, e.g.,... 

Surface complex formation with ligands (anions) as a function of pH... 

Fig. 2 Chemical structures of fluorescent ligands (anion coordination sites are indicated in blue) and tentative reaction schemes on interaction of 6 with chloride and fluoride in dichloromethane according to the color patterns observed in [41]. The wavelengths of absorption given in brackets have been calculated for the geometry optimized species 6, 6-C1 and 6 in the gas phase by semiempirical AMI calculations (Ampac V6.55, Semichem)...

Complexation reactions are assumed to proceed by a mechanism that involves initial formation of a species in which the cation and the ligand (anion) are separated by one or more intervening molecules of water. The expulsion of this water leads to the formation of the inner sphere complex, in which the anion and cation are in direct contact. Some ligands cannot displace the water and complexation terminates with the formation of the outer sphere species, in which the cation and anion are separated by a molecule of water. Metal cations have been found to form stable inner and outer sphere complexes and for some ligands both forms of complexes may be present simultaneously. 

In the reaction below a metal ion reacts with n ligand anions h to form an uncharged complex ML. If the ions dissolve only in the aqueous phase, and the metal complex and undissociated acid HL dissolve only in the organic phase, lUPAC allows the reaction to be written in four different ways ... 

When the ligand anions are monodentate, the donor coordination site employed must be established. 

Solution studies on the compounds [BU4N] [M(R2NCS2)3] (M = Zn or Cd R = Me or Et) show the zinc complex (unlike the cadmium complex) to be some 90% dissociated into the neutral bis complex and the free ligand anion. This is in accord with the structure of the solid complex,907 which shows that only one dithiocarbamato group is bidentate, the other two being formally unidentate. 

If CL, CM and pH are kept the same for a series of different cations, the position of equilibrium in equation (6a) would depend on the value of the formation constant, (3 , for the metal complex in question and the extent of precipitation on its solubility product, K. The more stable the complex and the lower its solubility in water, the greater the extent of precipitation. Essentially there is a competition between cations M"+, and protons, H+, for the free ligand anion Ox. ... 

Stufkens and Hard observed light induced formation of zwitterions and biradicals from the diimine clusters [Os3(CO)io(iPr-AcPy)] and [Os3(CO)io (dmb)] using picosecond UV/vis and nanosecond IR spectroscopies. Picosecond transient spectra indicated generation of the ligand anion radical and evolution over approximately 50 ps to a much longer lived transient that was attributed to an opened cluster with an associated ligand on the diimine metal. Figure 15 shows the proposed evolution of species in the photoreaction [93,94]. 

Figure 7.11. Dissolution rate dependence on the presence of organic ligand anions (pH 2.5-6) can be interpreted as a linear dependence on the surface concentrations of deproton-ated ligands, [ —L] jy (nmol m 2 h ) is that portion of the rate that is dependent on surface complexes only. In the case of citrate and salicylate, at pH 4.5 corrections accounting for the protonation of the surface complexes were made. [From Stumm et al. (1985), with permission.]...

The affinity of the various calix[4]pyrrole receptors (1,2, 8-aa/ / ) for the fluoride anion in acetonitrile at 298.15 K is reflected in the conductometric titration curves of fluoride against the ligand/anion ratio. The data shown in Fig. 4b are in accord with 1H NMR studies which show (Table 1) that the most significant chemical shift changes in the resonances of pyrrole NH and / -CH protons of receptors 1 and 2 were observed upon complexation with the fluoride anion in acetonitrile. 

Like calix[4]arenes, calix[4]pyrroles are versatile ligands to the extent that the composition of the anion receptor complex is solvent dependent. A representative example is that involving 8 and the fluoride anion. As shown in Fig. 4b, the well-defined change in curvature observed at 1 1 ligand fluoride mole ratio indicates that in acetonitrile one fluoride anion interacts per unit of receptor 8. However, in moving from acetonitrile to A/,A/-dimethylformamide, the noticeable changes in curvature observed at a ligand/anion mole ratio of 0.5 and 1, indicate respectively the formation of a 1 2 and 1 1 anion complexes, respectively, in this solvent. 

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