Elements of the Fifth Main Group

Structure of a-selenium. Left side view of a helix with 3j 2 screw symmetry. Right view along the helices the unit cell and the coordination about one atom are plotted 

Tellurium crystallizes isotypic to a-selenium. As expected, the Te-Te bonds in the chain (283 pm) are longer than in selenium, but the contact distances to the atoms of the adjacent chains are nearly the same (Te- Te 349 pm). The shortening, as compared to the van der Waals distance, is more marked and the deviation from a regular octahedral coordination of the atoms is reduced (cf. Table 11.1, p. 111). By exerting pressure all six distances can be made to be equal (cf. Section 11.4). 

Two modifications are known for polonium. At room temperature a-polonium is stable it has a cubic-primitive structure, every atom having an exact octahedral coordination (Fig. 2.4, p. 7). This is a rather unusual structure, but it also occurs for phosphorus and antimony at high pressures. At 54 °C a-Po is converted to /3-Po. The phase transition involves a compression in the direction of one of the body diagonals of the cubic-primitive unit cell, and the result is a rhombohedral lattice. The bond angles are 98.2°. 

For solid nitrogen five modifications are known that differ in the packing of the N2 molecules. Two of them are stable at normal pressure (transition temperature 35.6 K) the others exists only under high pressure. At pressures around 100 GPa a phase transition with a marked hysteresis takes place, resulting in a non-molecular modification. It presumably corresponds to the a-arsenic type. Electrical conductivity sets in at 140 GPa. 

Phosphorus vapor consists of tetrahedral P4 molecules, and at higher temperatures also of P2 molecules (P=Pbond length 190 pm). White phosphorus forms by condensation of the vapor it also consists of P4 molecules. Liquid phosphorus normally consists of P4 molecules, but at a pressure of 1 GPa and 100 °C polymeric liquid phosphorus is formed which is not miscible with liquid P4. 

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Lothar Weber was born in 1944 in Langenols in Schlesien. He studied at the Universitat Marburg and received his doctorate there under the direction of Professor Gunter Schmid in 1973. Thereafter he carried out postdoctoral studies with Professor Barry M. Trost at the University of Wisconsin in Madison, USA. On his return to Marburg, he began the experimental work leading to his habilitation, which was completed in 1982 at the Universitat Essen. His work focused on the coordination chemistry of sulfur ylides. In 1985, he became a C2 Professor and then joined the Fakultat fiir Chemie der Universitat Bielefeld. His research interests include the chemistry of compounds with low-coordinate elements of the fifth main group, the synthesis of homo- and heterocycles with heavy elements, as well as new aspects in boron chemistry. 

The growth of inorganic chemistry in recent years has led to a substantial increase in our knowledge of the halogen compounds of the elements of the fifth main group of the periodic table. These compounds are usually sensitive to hydrolysis and have to be handled under conditions such that water is excluded. It is understandable, therefore, that special techniques associated with the study of nonaqueous solvents have made a major contribution to progress in this field. 

All reactions with XeF MF5" (M = As, Sb) mentioned so far for elements of the fifth and sixth main group have been examples of either oxidative fluorination forming E-F cations or the synthesis of a transition state with an E-XeF+ structural unit. It seemed logical to extend our investigations to the elements of the seventh main group in order to prepare Hal2F or possibly Hal2XeF salts. However, the analytical and... 

The molecules in crystalline chlorine, bromine and iodine are packed in a different manner, as shown in Fig. 11.1. The rather different distances between atoms of adjacent molecules are remarkable. If we take the van der Waals distance, such as observed in organic and inorganic molecular compounds, as reference, then some of the intermolecular contacts in the b-c plane are shorter, whereas they are longer to the molecules of the next plane. We thus observe a certain degree of association of the halogen molecules within the b-c plane (dotted in Fig. 11.1, top left). This association increases from chlorine to iodine. The weaker attractive forces between the planes show up in the plate-like habit of the crystals and in their easy cleavage parallel to the layers. Similar association tendencies are also observed for the heavier elements of the fifth and sixth main groups. 

The layer and chain structures of the elements of the fifth and sixth main groups result by contraction of certain distances in the a-Po structure (stereo images)... 

Elements of the Fifth and Sixth Main Groups underpressure... 

Crystal structure determinations from very small samples have become possible due to the high intensities of the X-rays from a synchrotron. Very high pressures can be exerted on a small sample situated between two anvils made from diamond. In this way, our knowledge of the behavior of matter under high pressures has been widened considerably. Under pressure the elements of the fifth and sixth main groups exhibit rather unusual structures. A synopsis of the structures that occur is given in Fig. 11.9. 

Arsenic, antimony, and bismuth are the metallic representatives among the fifth main-group elements. Owing to early chemotherapeutic applications a tremendous number of organo-metal derivatives has been prepared, especially of arsenic. 

Ditellurides, also in the fifth period, seem quite analogous to distibines. Like tetraphenyldistibine (1) the red diphenylditelluride (56) does not associate in the solid state. The closest intermolecular Te---Te contact is 4.255 A, near the van der Waals separation of 4.40 A 61). On the other hand, di(p-methoxyphenyl)ditelluride (57), which has a brown-green metallic luster in the solid, has close intermolecular Te---Te contacts of 3.57 and 3.98 A (62). The ratio Te---Te/Te—Te is 1.32. Just as in the distibines the intermolecular bonding in ditellurides is sensitive to substitution. It is also interesting to note that the intermolecular interaction in ditellurides and dihalogens occurs normal to the metal-metal axis, as well as colinear as in distibines (63). Thus, it is clear that the intermolecular association shown by distibines is a general property of many of the diatomic like compounds of the heavier main group elements. 

Arsenic, an element with metallic and nonmetallic properties, has the atomic number 33, the atomic weight 74.9216, and belongs to group 15 (formerly the fifth main group) of the periodic table. Arsenic shows valencies of 5+, 3+, and 3- in its compounds. Natural As consists of the stable isotope 75. Radioactive isotopes exist with masses fiom 67 to 86 and half-life times between 0.9 sec and 80.3 days. 

Bismuth is a metallic element with atomic number 83 and atomic weight 208.98. It belongs to group IS (formerly the fifth main group) of the periodic table and generally shows a valence of 3+ but also of 5+, 4+, and 2+ in its compounds. Natural Bi consists of the stable isotope 209. Artificial isotopes are known with masses tetween 199 and 215 and half-lives from 2.15 min to 3 million years. 

Materials for PC Media. Crystalline alloys of elements from the fifth and sixth main group are preferred (3,103,109—111). As the first PC materials, tellurium suboxides as well as Te/Se or Te films that had been doped with small amounts of other elements like Ge, As, or Sb to shift the crystallization point to >100°C have been described. 

As early as 25 years ago, a comprehensive overview [8] of the chemistry of ferrocenophanes was published. In this particular field, development is so extensive that in a recent survey [9] only certain aspects were considered. In this progress report we will restrict ourselves to the complexation chemistry of cyclophanes with transition metals. Complexation chemistry with main group metals mainly consists of structural descriptions at present. As to complexes of cyclophanes with elements of the third [10], fourth [10a, 11] and fifth [12] main groups we refer to the references given. 

We shall not discuss the electron configurations of all the elements in the fifth and sixth periods, but be content to point out that the ground state electron configurations of all main group elements in Groups 1, 2 and from 12 to 18 are in accord with the aufbau model. 

A rule of thumb, based on observation, is that the need for electron correlation becomes more important as one descends to the heavier main group elements and toward the right in the first transition series.This observation can be rationalized in terms of weaker bonding for heavier MG elements as well as first row TMs, hence lower energy excited states and a greater electron correlation contribution (see Eq. [2]). One final point is that quantum mechanical methods for including correlation scale as the fifth to seventh power of N, where N is the number of basis functions. 

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