Ligands penta-dentate

The reaction of hexacyanometalates with metal complexes chelated by penta-dentate ligands may afford polynuclear complexes. The presence of the penta-dentate ligand precludes the polymerization that leads to extended systems. The preparation of a representative heptanuclear, mixed-valance iron complex, [Fe (CNFe° (salmeten))6]Cl2 6H20, is detailed herein. 

Table 50 Selected Complexes with Tetra- and Penta-dentate Ligands Containing Heterocyclic Nitrogen Donors...

Figure 5-82. The ring opening of both thiazoline rings in 5.37 leads to the formation of a penta-dentate N2S2 ligand.

Penta- and Hexa-dentate Ligands. Perhaps the best known hexadentate ligand is ethylenediaminetetraacetate, EDTA4-, which can also be penta-dentate as EDTAH3-. 

Considering the denticity of ligand products typically assembled, the most often encountered are tetradentate macrocycles, followed by hexadentate species. Potentially tridentate macrocyclic products are described for nickel(II), copper(II), bor-on(III) and molybdenum(O) silver(I) and mercury(II) promote assembling penta-dentate and hexadentate macrocycles thallium(I), strontium(II), lanthanum(III) and the lanthanides(IIl) from Ce to Gd (Pm was not studied bccau.sc of its radioactivity) hexadentate products and the metal ions from Tb to Lu promote the formation of tetra- and hexa-dentate macrocyclic ligand products. Variable denticity of synthesised systems is conunon to most first-row transition elements as well as to alkaline metal ions which serve mainly to form crown ethers and related compounds. 

Tetra- and penta-dentate ligands are constructed mainly by using cations of the first-row transition elements, which manifest template functions in accordance with the stereochemical preferences of the appropriate metal ions. 

Table 52 Selected Nickel(II) Complexes with Tetra-, Penta-, and Hexa-dentate Schiff Base Ligands... 

Selected examples of tetra-, penta- and hexa-dentate Schiff base ligands and of the corresponding nickel(II) complexes are reported in Table 52. 

In Table 108 significant examples of nickel(II) complexes with mixed-donor macrocycles of various denticity are reported. Apart from the nitrogen atoms, the heteroatoms in the macrocyclic rings are usually either O or S, or in a few cases P. Few examples of nickel complexes with macrocycles containing all-sulfur or all-phosphorus donor atoms have been reported to date they are also included in Table 108. In nickel(II) complexes formed by mixed-donor penta- and hexa-dentate ligands the oxygen atoms of the macrocycle are often only weakly coordinated or are not coordinated at all. 

Preti et al. (1502) showed that cyclic dithiocarbamates complexes [Ni(S2CNC4HgX)2] (X = 0, S, NH, NMe, CH2) react with 3 equiv of en to give diamagnetic products, [Ni(S2CNC4HgX)2(en)3]. Their stmctures are unknown, but formation of salts, [Ni(en)3] [S2CNC4HgX]2, have been ruled out. The authors favor a penta coordinate, high-spin, nickel(II) center with mono-dentate ethylenediamine and dithiocarbamate ligands. 

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