Ligands anionic polydentate

In contrast to the facile reduction of aqueous V(III) (—0.26 V versus NHE) [23, 24], coordination of anionic polydentate ligands decreases the reduction potential dramatically. The reduction of the seven-coordinate capped-octahedral [23] [V(EDTA)(H20)] complex = —1.440 V versus Cp2Fe/H20) has been studied extensively [25,26]. The redox reaction shows moderately slow electron-transfer kinetics, but is independent of pH in the range from 5.0 to 9.0, with no follow-up reactions, a feature that reflects the substitutional inertness of both oxidation states. In the presence of nitrate ion, reduction of [V(EDTA) (H20)] results in electrocatalytic regeneration of this V(III) complex. The mechanism was found to consist of two second-order pathways - a major pathway due to oxidation of V(II) by nitrate, and a minor pathway which is second order in nitrate. This mechanism is different from the comproportionation observed during... 

Synthesis of Complexes Containing Anionic Polydentate Ligands, NiR(chelate)L 52... 

Complexation is a phenomenon that involves a coordinate bond between a central atom (the metal) and a ligand (the anions). In a coordinate bond, the electron pair is shared between the metal and the ligand. A complex containing one coordinate bond is referred to as a monodentate complex. Multiple coordinate bonds are characteristic of polydentate complexes. Polydentate complexes are also referred to as chelates. An example of a monodentate-forming ligand is ammonia. Examples of chelates are oxylates (bidentates) and EDTA (hexadentates). 

As ligands in metal complexes, the anions derived from these acids display a broad variety of coordination patterns (Scheme 2). Usually, the ligands are described as monodentate, bidentate chelating or bridging, and polydentate. We use here a description based upon the number of connections between the... 

Tri- and tetrametaphosphimates can act as polydentate ions. In principle, coordination by both nitrogen and oxygen is possible, but as seen in the salts of monovalent cations, almost exclusively coordination via oxygen is expected. Thus, the anions can act as polydentate and/or bridging ligands. The trimetaphosphimate ion can act in various ways ... 

Among the polydentate carbene ligands, particular interest has recently been placed on cyclic polycarbenes. ImidazoUum precursors like 23 [89] or 24 [90, 91], which upon C2 deprotonation would lead to tetradentate or even hexadentate double-pincer NHC ligands, have been prepared. Their interesting coordination chemistry will be discussed in Sect. 4. Finally, Arnold et al. developed and reviewed NHC ligands which are functionalized with additional anionic (alkoxide or amido) donor groups [92]. 

The complexes of alkali metal ions and of their salts described in paras. II—V may also be considered as lattice compounds because they do not necessarily persist in solution. Where the charge on the cation is neutralised by a small anion to give a salt, the solid may contain ion pairs coordinated by the additional ligand molecules, or the ions may be separated by the ligands, which usually form hydrogen bonds to the anion. When the cation is neutralised by a polydentate anion, the co-... 

Nickel forms a large number of complexes with various anions (monoden-tate, bidentate, and polydentate) and many neutral ligands. The most common coordination numbers of the metal in these complexes are six and four while the metal is usually in +2 oxidation state, Ni2+. Also, some complexes of three and five coordinations exist. Several zero valent nickel complexes, such as nickel tetracarbonyl, and a number of substituted carbonyl complexes are well known. 

L = aromatic amines, picolinic acids, picolinate N-oxido anions, dicarboxylic acids, O and N containing polydentate ligands etc. 

EDTA is a common polydentate ligand. In EDTA, the hydrogen atoms are easily removed in solution to produce anionic EDTA4. In its anionic form EDTA has six binding atoms, two nitrogen and four oxygen as depicted in Figure 39.1. 

In the available literature, only six PBP structures of Ni(II) have been reported so far (34,38,40,45,48,50) and four of them include acyclic pentadentates (XVII-XX). These pentadentates are equato-rially coordinated in their neutral form with weak conformational flexibility. Apical positions are always occupied by water molecules, while nitrate anions always represent counter ions in the structure. The change of any of the mentioned structural elements (polydentate, axial ligands, or counter ion) affords complexes of which the structure has not been determined (52,54,56,58). These are complexes with the same type of ligands XX-XXIII, containing Cl- ions and water molecules, but it is still unknown which of these two monodentate ligands occupies the apical positions. It is questioned whether both monodentates actually occur in the coordination sphere, and because of that it is impossible to discuss the geometry of these complexes in more detail. 

A diverse range of acyclic polydentate ligands has been used to form complexes with alkali and alkaline earth metal cations. For convenience of description these ligands have been subdivided into anionic and neutral species. 

In the presence of anions such as phosphate and oxalate, the relaxivity of [Gd(Tex)]2+ is considerably reduced revealing that these anions compete with water for binding to the Gd3+ ion [167,171]. Most likely, texaphyrin complexes self-associate due to strong van der Waals interactions. UV-VIS studies suggest that the aggregates dissociate upon interaction with polyuronides (pectate, alginate), which probably act as polydentate polycarboxylate ligands for the [Gd(Tex)]2+ complexes [173]. 

Tetrazoles exhibit qualities of acids, bases, acceptors of hydrogen bonds (cf. Section 6.07.4.5), and polydentate ligands (cf. Section 6.07.5.3.4). NH-LJnsubstituted tetrazoles behave both as substrates and intermediates in transacylation processes (cf. Section 6.07.5.4), etc. Tetrazolate anions (tetrazolides) possess high aromaticity and reactivity toward electrophilic reagents (cf. Sections 6.07.4.1 and 6.07.5.3.2). The thermal and photochemical decomposition of tetrazoles involves formation of nitrenes and other intermediates of high reactivity (cf. Sections 6.07.5.2 and 6.07.5.7) These properties provide a possibility of use tetrazoles as catalysts in chemical and biochemical reactions. 

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