Coordination rare tetrahedral

In their pursuit of modeling Type I copper proteins, Kitajima et al. reported112 a rare, tetrahedrally coordinated complex (105), which displayed an EPR spectrum consistent with the presence of the unpaired electron in the dz2 orbital.1 They also isolated a square-pyramidal DMF adduct (complex (106)). They were successful in providing structural proof of a copper(II) complex (trigonal pyramidal) with C6F5S -coordinated complex (107), with CuN3S chromo-phore.113 The X-ray analysis (poor data set) of a closely similar complex with Ph3CS as the... 

Halide complexes are also well known but complexes with nitrogen-containing ligands are rare. An exception is the blue phthalocyanine complex formed by reaction of Be metal with phthalonitrile, 1,2-C6H4(CN)2, and this affords an unusual example of planar 4-coordinate Be (Fig. 5.5). The complex readily picks up two molecules of H2O to form an extremely stable dihydrate, perhaps by dislodging 2 adjacent Be-N bonds and forming 2 Be-O bonds at the preferred tetrahedral angle above and below the plane of the macrocycle. 

The coordination number of Ni rarely exceeds 6 and its principal stereochemistries are octahedral and square planar (4-coordinalc) with rather fewer examples of trigonal bipyramidal (5), square pyramidal (5), and tetrahedral (4). Octahedral complexes of Ni arc obtained (often from aqueous solution by replacement of coordinated water) especially with neutral N-donor ligands such as NH3, en, bipy and phen, but also with NCS, N02 and the 0-donor dimethylsulfoxide. dmso (Me2SO). 

The molecular structure of the f-butyl derivative [BpBut]ZnBul has been determined by an x-ray diffraction study, which confirms a distorted trigonal planar coordination environment for zinc, as shown by the two views in Figs. 26 and 27 (80). In comparison to tetrahedral coordination, three-coordinate zinc alkyl complexes are rare (84). 

A rare example of thiourea coordination to low-valent Co is of a disubstituted thiourea as bridging ligand, observed in the cluster Co3(CO)7(/i3-S)(/i- 72-PhNC(S)NHCH2Ph) which is formed by reaction of Co2(CO)8 with the thiourea.172 The crystal structure of the product defines a tetrahedral Co3S core with all carbonyls in terminal positions and the deprotonated thiourea bridging two Co centers via the S and an amido N. 

The tetrahedral Cu04 group is rarely observed and no strong evidence has ever been presented for this highly symmetrical environment about the divalent copper ion. Coordinations higher than VI (or IV + II) are also very rare. Statistical data for Cu2+ coordination environments in 234 oxysalts and 75 minerals will be summarized. 

Although Ti " " is almost always octahedrally coordinated, it is often found displaced from the centre of its coordination sphere, as for example in BaTiOs (23759, Fig. 10.4), where, even though the distortion is driven by the lattice strains discussed in Chapter 12, it is stabilized by a favourable electronic state. Even when Ti " " possesses a crystallographic centre of symmetry as in SrTi03 (201256), it is probable that it is dynamically distorted (Abramov et al. 1995). By contrast, Sn, which is similar in size to Ti, is found in octahedral environments that are rarely distorted. Tetrahedral Ti is known in Ba2Ti04 (2625) but in very few other compounds. 

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