Trigonal coordination

The series M(PP)C1 (PP = 2,ll-bis(diphenylphosphinomethyl)benzo[c]-phenanthrene M = Cu, Ag, Au) is interesting in showing group trends (Table 4.7) where the silver and copper complexes have trigonal coordination and the gold member is linear [50]. 

As discussed in Section II. A, theoretical studies predicted that phospha-silenes with silyl substituents attached to phosphorus should have planar, trigonally coordinated silicon, with the Si—P w-bonds strengthened by the hyperconjugative influence of the silyl group.16 Recently, this was proved by a single-crystal X-ray structure determination of the derivative... 

A second type of defect is associated with boron or aluminum impurities that are present in SiCh- In porous glass Muha (129) observed a rather complex spectrum which results from hyperfine interaction with 10B and UB isotopes. The spectrum is characterized by g = 2.0100, g = 2.0023, an = 15 and a a. = 13 G for nB. The paramagnetic defect is apparently a hole trapped on an oxygen atom which is bonded to a trigonally coordinated boron atom. This center is irreversibly destroyed upon adsorption of hydrogen. 

Metallocryptands, based on trigonally coordinated Pt(0) or Pd(0) with 2,9-fc(diphenylphosphino)-l,10-phenanthroline possessing PjPd bridge-heads, binds Pb(II) within the structure . 

However, we should emphasize that the NBO/NRT concepts of hybridization, Lewis structure, and resonance differ in important respects from previous empirical usage of these terms. In earlier phases of valence theory it was seldom possible to determine, e.g., the atomic hybridization by independent theoretical or experimental procedures, and instead this term became a loosely coded synonym for the molecular topology. For example, a trigonally coordinated atom might be categorized as sp2-hybridized or an octahedrally coordinated atom as d2sp3-hybridized, with no supporting evidence for the accuracy of these labels as descriptors of actual... 

Intermediate between the extended four-coordinate connectivities that dominate the low- T solid phase and the two-coordinate ring/chain connectivities that dominate at higher T are certain //jree-coordinate polyhedral structures that retain a degree of cooperative proton ordering. Two examples of such trigonally coordinated buckyball clusters, a 24-mer and a 28-mer, are shown in Fig. 5.31. The... 

Both Zn and Cu alloy with metaUic Fe. In silicates, Ctf and Zrf substitute for Fe and Mg in octahedral sites but the octahedral preference energy is small for Cu and zero for Z4, which, beyond their trace abundance in nature, explains why these two elements are not particularly abundant in any major FeMg-silicates. Cu and Zn form a very large number of sulfides (often in association with Fe) with Cu and Cu occupying either octahedral or tetrahedral sites. They also form scores of carbonates and hydroxides. Cu in chalcopyrite and Zn in sphalerite are tetrahedrally coordinated with S but other sulfides may contain trigonally coordinated metals. Cu in malachite and Zn in smithsonite are octahedrally coordinated with O. 

As a consequence of its electronic configuration, a variety of coordination numbers and geometries have been observed for copper(I) compounds, especially for inorganic representatives (see Fig. 1.3) [32]. In the organometallic chemistry of copper, the linear and trigonal coordination geometries in particular, though distorted towards T-shaped, are frequently encountered. 

The Jensen symbols are very important in helping to unravel the relationship between the different structure types in neighbouring domains. For example, it is not fortuitous that the NaCl and NiAs domains adjoin each other. Their Jensen symbols 6/6 and 8IV/6 tell us immediately that in NaCl the Na and Cl sites are octahedrally coordinated, whereas in NiAs the Ni site is octahedrally coordinated (but with two extra capping atoms), and the As site is trigonally coordinated. It is also not surprising that at the boundary between cF8 (NaCl) 6/6 and hP4(NiAs) IV/6 we find the two much smaller domains of hP8(TiAs) 7/6, 6 and tI8(NbAs) 6 /6. Nor is it unexpected to find the two islands of oP8(MnP) 10 78 " stability in the hP4(NiAs) 8rv/6 domain. A distorted NiAs structure type, MnP leads to the bicapping of the trigonal prismatic coordination about the As site, that is 6 - 8W (cf Fig. 1.9). Further, we see that the cP8(FeSi) 13713 domain adjoins a cP2(CsCl) 14/14 domain they are related structure types as mentioned earlier. 

A series of pyridoxal amino acid Schiff base complexes have been prepared in which Al is trigonally coordinated by N and two O atoms.415 These provide a model for the intermediate in the pyridoxal-catalyzed reactions of amino acids. Some Schiff base complexes produced in reactions of the bases with Al(OPri)3 have been assigned a structure with five-coordinate aluminum.416417... 

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