Least coordinating anion, formation from

Exactly the same problem arises with the recent studies of NiO solubility by Tremaine and Leblanc (25) and again the thermodynamic data on the aqueous anionic species at 300 C are likely to be more reliable than on the Ni + ion. There is good spectroscopic evidence for complex formation in chlorides of nickel (II), (26) cobalt (II) (27), and copper (II) (28) at 300°C and above. Most of the work was done at rather high Cl concentrations but qualitatively the effects of dielectric constant and concentration are as expected. A noteworthy feature (which estimation procedures will have to allow for) is the change from 6 to 4 coordination at the lower pressures (150-300 bar) and the higher Cl concentrations. This change appears to take place with only 2 or 3 Cl ions coordinated to the metal (at least in the case of Ni(II)). 

At least two different Pd(0) species can be involved in both the oxidative addition and n-coordination steps, depending on the anions and ligands present. High halide concentration promotes formation of the anionic species [PdL2X] by addition of a halide ligand. Use of trifhioromethanesulfonate anions promotes dissociation of the anion from the Pd(II) adduct and accelerates complexation with electron-rich alkenes. 

By comparison with the mercury(I) and mercury(II) ions (Chapter 56.1), the coordination chemistry of (10) and (11) has received little attention. Both ions disproportionate to Hg° and Hg2+ /Hg2+ in media more basic than those from which they can be prepared. However, the existence of (11) (which can be regarded as a complex of (10) with Hg°) and the cation-anion coordination found in solid salts of (10)38 suggest that this ion, at least, might form stable complexes with suitable weak donors. In addition, the formation of as yet incompletely characterized Hg2BrCI04 2SnBr2, which may contain both Hg—Hg and Hg—Sn bonds,42 and the isolation and characterization of [ (np3)Co HgHg Co(np3) ] (1 see Section 11.2) may presage a wider occurrence of catenated heterometallic polymercury species. Slow disproportionation of (11) into (10) and Hg3-X(MF6) occurs even in liquid S02.39 As discussed below, there is evidence for mercury atom transfer between (10) and (11) in liquid S02. 

It is clear from the examples reported that carbon monoxide, when coordinated to a metal in a neutral complex, is not sufficiently activated to react with organic nitro compounds under mild conditions. More precisely, the first act of this reaction is the electron transfer from the metal to the nitro group to give a radical couple and this requires a very basic metal. This explains why basic ligands usually activate transition metal carbonyls in these catalytic reactions. Moreover, basic ligands such as Bipy favor the in-situ formation of the [Rh(CO)4] species from rhodium clusters. The effect of co-catalysts such as halide anions is more subtle, but even the action of these might, at least in part, be directed toward an increase of the electron density of the metal. 

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