Trigonal structures

Mercuric sulfide (HgS) is dimorphic. The more common form, cinnabar (red a-form), has a distorted RS, trigonal structure which is unique among the monosulfides, for the crystal is built of helical chains in which Hg has two nearest neighbors at 2.36 A, two more at 3.10 A, and two at 3.30 A. Bulk a-HgS is a large-gap semiconductor (2.1 eV), transparent in the red and near IR bands. The rare, black mineral metacinnabarite is the 3-HgS polymorph with a ZB structure, in which Hg forms tetrahedral bonds. Upon heating, 3-HgS is converted to the stable a-form. The ZB structure of HgS is stabilized under a few percent admixture of transition metals, which replace Hg ions in the lattice. 

Of particular importance in structural chemistry is the concept of hybridization, that is, the construction of linear combinations of atomic orbitals that transform according to the symmetry of the structure. For the present, a simple illustration is provided by the hybridization of atomic orbitals in a molecule or complex ion of trigonal structure. 

Figure 3.3 Bonding structures for different carbon materials (a) diamond, (b) graphite, (c) carbon nanotubes and (d) fullerenes. Scheme of the pyramidalization angle (0p) in deformed sp bonding in comparison with a trigonal structure.

There is no general theoretical study for trialkyl-substituted cations R3E, which investigates the relationship of the classical planar trigonal structure to isomeric complexes RE /R2 and its relative energy compared to the dissociation products, the singly coordinated four-valence-electron species R E and the hydrocarbon R2. The only exceptions are 7-norbornadienyl cations 37 for which the germyl and silyl cation has been intensively studied theoretically by Radom and Nicolaides. ... 

An intermediate, often transient, appearing in a chemical or enzymatic reaction in which a carbon atom, which had been double-bonded (i.e., in a trigonal structure) in a particular molecular entity, has been transformed to a carbon center having a tetrahedral arrangement of substituents. Tetrahedral intermediates of proteases have been stabilized with cryoenzymological tech-niques ... 

Corundum has a trigonal structure. The oxygen ions are arranged in approximately hexagonal closed packing. Between the oxygen layers there are sites... 

In addition to the three known natural gas hydrates, several other hydrate structures exist. Dyadin et al. (1991) found four hydrate structures and Jeffrey (1984) proposed five additional hydrate structures. These structures have yet to be confirmed in natural gas systems, although new hydrate structures have been identified using x-ray diffraction such as a tetragonal structure for bromine (Udachin et al., 1997b), a trigonal structure for dimethyl ether (Udachin et al., 2001a),... 

A carbanion is closely related to the corresponding derivative of nitrogen the methyl anion is isoelectronic with ammonia itself. By analogy with the configuration of amines, carbanions should also possess a pyramidal structure (I) in which the pair of electrons of the carbanion is placed in an sp3 orbital, rather than the alternative trigonal structure (II) with the pair of electrons in a p orbital. 

The boronhalides (BX3) are volatile, highly reactive, monomeric compounds, which show no detectable tendency to dimerize. In this they resemble organoboranes, BR3, but differ sharply from diborane, B2H6. The molecules have a planar trigonal structure with the boron atom (sp2 - hybridization) in the centre. This class of compounds also exhibits partial double bond character due to pT - px interactions of the empty pz-orbital of boron and the filled pz-orbital of the halogen. Because of the incomplete octetstructure of the central boron atom, borontrihalides behave as strong Lewis acids. 

Structural work on UO2F2 (24,246,247) has gradually been refined over the years, and a neutron diffraction study has shown that UO2F2 has a trigonal structure in which the uranium is eight-coordinate... 

Trigonal geometry with sp2 hybrid orbitals, ffybridization of an s orbital with two p orbitals gives a set of three sp2 hybrid orbitals. This trigonal structure has bond angles of about 120°. The remaining p orbital is perpendicular to the plane of the three hybrid orbitals. 

The AnH2 (An = Ac, Np-Bk) have the fluorite structure. Thft2 is unique in that it has tetragonal symmetry. In /3-UH3, the metal atoms have a coordination number of 12 and the H atoms occupy tetrahedral intersites. For the other AnH3 (An = Np Bk), the crystals have a trigonal structure. 

Mixtures of NO/O3/O2 also produce NO4+ ions under the conditions described above, but their decomposition products indicate an O3NO coimectivity. Three isomeric NO3+ cations have been detected in the mixtures described above under different conditions one has O2NO connectivity and two share O NO2 coimectivity, but with different structures. One NO3+ cation has a trigonal structure like the radical NOb-, the other corresponds to a weakly bound species that undergoes a near threshold decomposition and is assumed to have ONOO connectivity. Neutralisation-reionization mass spectrometry of this NO3+ isomer shows weak evidence for a neutral ONOO isomer (69) with a lifetime exceeding 1 ts. 

The purple molybdenum bronzes have the general formula AMoeOn, where A = Li, Na, K or Tl. The potassium and thallium members have trigonal structures while the Li and Na compounds show different types of distortions, which result in monoclinic symmetry. The ideal value of A in this structure should be 1.0 however, it seems to be closer to 0.9 for all members except Tl. Only for the Na compound has a slight stoichiometry range (0.84 < x < 0.96) been observed. The origin of this behavior is not clear although it may be electronic in nature. 

Why does a Cu(II) ion with the ligands in the blue copper proteins assume a trigonal structure, whereas most inorganic cupric conplexes are tetragonal (square planar, square pyramidal, or distorted octahedral) [63,64] We have faced this question by optimising the geometry of a number of models of the type... 

---