Anionic ligand, charge separation

If we now consider the CO/H2 system, then the catalytic process can be envisaged as a combination of (1) and (2) above, carbon monoxide being activated by coordination and hydrogen by addition. If we adopt the view that in a transition metal hydride the hydrogen is present as an anionic hydride ligand ( H ) (49) then, given the charge separation... 

Metal-substituted hemoglobin hybrids, [MP, Fe " (H20)P] are particularly attractive for the study of long-range electron transfer within protein complexes. Both photoinitiated and thermally activated electron transfer can be studied by flash excitation of Zn- or Mg-substituted complexes. Direct spectroscopic observation of the charge-separated intermediate, [(MP), Fe " P], unambiguously demonstrates photoinitiated ET, and the time course of this ET process indicates the presence of thermal ET. Replacement of the coordinated H2O in the protein containing the ferric heme with anionic ligands (CN , F , Nj ) dramatically lowers the photoinitiated rate constant, k(, but has a relatively minor effect on the thermal rate, kg. 

A secondary, more subtle, effect that can be utilized in the achievement of selectivity in cation exchange is the selective complexation of certain metal ions with anionic ligands. This reduces the net positive charge of those ions and decreases their extraction by the resin. In certain instances, where stable anionic complexes form, extraction is suppressed completely. This technique has been utilized in the separation of cobalt and nickel from iron, by masking of the iron as a neutral or anionic complex with citrate350 or tartrate.351 Similarly, a high chloride concentration would complex the cobalt and the iron as anionic complexes and allow nickel, which does not form anionic chloro complexes, to be extracted selectively by a cation-exchange resin. 

With alkali metal cryptates, cations and anions are well separated (except for the KNCS complex of [2.2.1] in which the ligand is too small to effectively shield the cation). Indeed the cryptated cation may be regarded as a very large, spheroidal cation (—10 A in diameter) of low surface charge density. 

As noted above, the ability of ligands with multiple phenyl rings positioned to allow resonance stabilization in an anion makes them ideal candidates for DAIP. Recent reports have demonstrated this with the presentation of a family of heavy alkaline earth triphenyhnethanides, all of which occur as charge-separated complexes in the solid state. Reaction of dibenzylbarium or strontium with triphenylmethane in the presence of 18-crown-6 and HMPA leads to the isolation of the DAIP triphenylmethanide, the barium compound of which is shown in Figure 35. 

Decisive properties of the phosphinomethanides are their stability towards reduction, the separation of the negative charge of the anionic ligand (centered at the formal carbanion) from the electron rich E(II) (E = Si, Ge, Sn) center by the phosphane donor atoms, the thus generated additional electrostatic stabilization and the four-membered chelate ring formation [1]. 

Functionalization of Ceo with a porphyrin on one side and with a pyridyl ligand on the other side leads to the supramoleciflar triad 39 formed by a covalent-CO ordination approach (Fig. 31a) [18,20,24]. The photophysical properties of this system were studied in detail and show that energy-transfer occurs from the singlet excited state of the free-base porphyrin to the zinc(ll) porphyrin. A charge separated state with formation of the Ceo radical anion and the zinc(II) porphyrin radical cation is produced at an increased efficiency upon complex formation of the array. 

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