Octahedral coordination centers

Three slightly different octahedrally coordinated centers have been found in topaz by the EPR method (Schott et al. 2003), thus its participation in topaz luminescence is possible. 

Figure 26 A schematic representation of the tetrahedral supramolecnlar assembly M4L4 with fonr octahedrally coordinating centers linked by tripodal ligands containing three di-bidentate entities (a) examples of the tripodal ligands, (b)—(i), (ii)/ and (iii)7" ...

Cytochromes. A cytochrome is a protein containing a heme with an iron cation bonded to four donor nitrogen atoms in a square planar array. Figure 20-29a shows the structure of cytochrome c, in which a histidine nitrogen atom and a cysteine sulfur atom occupy the fifth and sixth coordination sites of the octahedral iron center. 

Diacetylpyridine bis(S-methylisothiosemicarbazone), 46, yields five-coordinate [Ni(46-H)]X (X = I, NCS) and six-coordinate [Ni(46-2H)] [203]. The latter compound is thought to be dimeric or polymeric with octahedral nickelfll) centers. 

The structures of [TpMe2]2Zn (80) and [Tpph]2Zn (85) have been determined by x-ray diffraction (Fig. 28). Interestingly, [TpMe2]2Zn and [TpPh]2Zn do not possess similar structures. Specifically, whereas the zinc center in [TpMe2]2Zn is six-coordinate and octahedral, the zinc center in [TpPh]2Zn is only four-coordinate and tetrahedral. Thus, the potentially tridentate [TpPh] ligand acts only as a bidentate ligand in [i72-TpPh]2Zn. However, it should be noted that the Fe in the related complex [TpPh]2Fe is octahedrally coordinated (90). 

X-ray techniques have shown the compound Ba5AlIr20n to have the two Ir centers in an octahedral coordination bound to A1Q4 tetrahedra, giving [Ir2AlOn]10 units.53... 

An imidazole/pyridyl ligand (4) coordinates to a copper(II) center and on deprotonation the imidazole reacts with bis(hexafluoroacetylacetonato)zinc to form a trimeric Cu2Zn species bridged by imidazoles. The zinc ion is in an octahedral coordination environment with Zn-N distances of 2.029(2) A.116... 

The reaction of ligand (),() -(A A- -dipicoliny 1 idcne) diazadiphenyldisulfide with zinc chloride forms [ZnClL]2[Zn2Cl6], where the zinc center has octahedral coordination with the ligand pentadentate with an N4S donor set.892... 

Hexacoordinated silicon takes benefit from the favorable octahedral arrangement of ligands around a coordination center. As expected, negatively charged substituents are superior in stabilizing high silicon coordination numbers. Most recently however, compounds with chelate ligands, mono anionic derivatives and compounds with more electropositive donors have been added to the series of penta- and hexacoordinated silicon species. 

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