Crystal structure Distortion derivatives

It is worth noting, however, that when the true crystal structure is derived from a minor distortion of a small unit cell, e.g. when the so-called superstructure has been formed or when the real structure is perturbed by a long period, small amplitude modulation function, many Bragg peaks and often the majority of them, may have extremely low intensity and become unobserved. These cases require special attention during the indexing, and detailed consideration of related subjects exceeds the scope of this book. 

There have been no reports of complexes of " JV-substituted thiosemicarbazones derived from 2-formylpyridine, but 2-acetylpyridine JV-methyl-thiosemicarbazone, 3a, formed [Fe(3a-H)2]C104 and [Fe(3a-H)2]FeCl4 [117]. The nature of these two species was established by partial elemental analyses, molar conductivities, magnetic moments, electronic, infrared, mass and electron spin resonance spectra. A crystal structure of a related selenosemicarbazone complex confirmed the presence of a distorted octahedral iron(III) cation coordinated by two deprotonated anions so that each ligand is essentially planar and the azomethine nitrogens are trans to each other the pyridyl nitrogen and selenium donors are both cis. 

The crystal structure of the C-functionalized imidazole derivative of 1,5,9-triazcyclododecane (75) shows a five-coordinate zinc with four /V-donors from ligand and chloride in a distorted trigonal bipyramidal arrangement. The Zn—N imidazole bonds are the shortest at 2.025(3) A.678 Deprotonation of the imidazole group resulted in a bridging imidazolate to form dinuclear zinc complexes. The pATa of 10.3 varies from the pATa of bound water with similar ligands (as low as 7.3) and the complex is not catalytic for the hydrolysis of esters. 

I shall take the simple view that most metal oxide structures are derivatives of a closest packed 02 lattice with the metal ions occupying tetrahedral or octahedral holes in a manner which is principally determined by size, charge (and hence stoichiometry) and d configuration (Jj). The presence of d electrons can lead to pronounced crystal field effects or metal-metal bonding. The latter can lead to clustering of metal atoms within the lattice with large distortions from idealized (ionic) geometries. 

The crystal structures of both (PPN)[M(CO)6] derivatives have been determined.717 The coordination polyhedron is octahedral (Nb—C 2.098(5) A CNbC 89.2(2)°). The PPN moiety is constrained to be centrosymmetric, and thus linear. These compounds correspond to the lowest oxidation state of niobium and tantalum for which structural data are available. A single v(CO) is found in the IR (1854 and 1852 cm-1 for Nb and Ta respectively). Comparable spectra are observed for Na[M(CO)6] in pyridine, but in solvents of lower dielectric constants such as tetrahydrofuran, additional bands attributed to distortion of the anion by the countercation are observed. [Nb(CO)6] appears to be the most labile carbonyl of the group VA analogs. 

A number of interesting modes of coordination are found with simple organic molecules as ligands, e.g. a crystal structure determination of the tetrameric methylzinc(II) derivative MeZn(OMe) 4684 shows the structure to be as in (101). Closely related to this is the structure of the polymerization catalyst Zn(OMe)2(EtZnOMe)6 (102).686 The centrosymmetric complex consists of two enantiomorphic distorted cubes which share a comer the zinc atoms occupy the comers of a tetrahedron and oxygen atoms the comers of an interpenetrating, but smaller, tetrahedron (Zn—O = 2.06 A). 

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