Anionic ligands five-coordinate complexes

Reaction of free-base porphyrin compounds with iton(II) salts in an appropriate solvent results in loss of the two N—H protons and insertion of iron into the tetradentate porphyrin dianion ligand. Five-coordinate iton(III) porphyrin complexes (hemins), which usually have the anion of the iton(II) salt for the fifth or axial ligand, ate isolated if the reaction is carried out in the presence of air. Iron(II) porphyrin complexes (hemes) can be isolated if the reaction and workup is conducted under rigorously anaerobic conditions. Typically, however, iton(II) complexes are obtained from iton(III) porphyrin complexes by reduction with dithionite, thiolate, borohydtide, chromous ion, or other reducing agents. 

On account of their diamagnetism and spectral properties the [NiX2(dpe)2] complexes have a substantial square planar structure in the solid state, possibly with two weakly interacting anions in axial positions, as found in Nil2(diars)2 (vide infra). In solution either five-coordinate or square planar species are formed.1277 3283,1347 The ligand tep (II) easily forms diamagnetic five-coordinate complexes [NiX(tep)2]Y in the solid state and in solution as well.1276,1281,1282 The structure of [NiI(tep)2]I is essentially square pyramidal.1281... 

The structure of the [Et4N]Cd[EtXant] 3 complex (346) has been determined (341). The anion is five-coordinate, with two EtXant ligands functioning as bidentate chelates and the third bonded to the Cd(II) center through one of its sulfur atoms. A tetragonal pyramidal coordination is seen for the CdS5 core, with the Cd(II) ion above the square base by 1.079(1) A (Table XI). 

Though the five-coordinate complexes, except for p-oxo dimers, coordinate sixth ligands, the association constants are low (K < 103 M-1) because of the strong trans effect of the anionic ligand (equation 13).18 In the presence of excess ligand, the anionic ligand may be displaced (equation 14).13... 

Answer. As we deal again with Mg2+ ions, the anion is formulated as [CoH5]4 with Co1 and five [H] ligands. Thus, it is an 18-electron compound. The limiting geometries adopted by five-coordinate complexes are trigonal bipyramidal (D3h symmetry and most common) for which Fe(CO)s provides an example and square pyramidal (C4v symmetry) for which [Ni(CN)s]3- provides an example, albeit slightly distorted. These, and distorted structures in between, have similar stabilities. 

If an extra anion binds to form a five-coordinate complex, it should become more difficult to dissociate a proton from bound water, so raising the p/Co and decreasing the catalytic efficiency. This theory has been tested using the pendant arm ligand L3 (Fig. 6), which forms a five-coordinate zinc complex [Zn(L3)(H20)]C104. An X-ray structure of the... 

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