Valence, Abnormal

Zinc and cadmium have the A 3 structure, but with abnormally large axial ratio (1-856 for zinc) instead of the value 1-633 corresponding to close packing. From the distances 2-660 A (for six bonds) and 2-907 A (for the other six) the bond numbers 0-54 and 0-21 have been deduced. If the axial ratio were normal each of the twelve bonds formed by a zinc atom with v = 4-5 would have bond number f. The assumption of the distorted structure permits a split into two classes with the more stable bond numbers and (or, probably, with the average valency of zinc equal to 4). 

Rare-earth ions inserted in the tetraborides have the 34- oxidation state, except for CeB4 and YbB4 (see Fig. 2). The abnormal volume contraction for the CeB4 unit cell can be explained by the presence of some Ce ions . Recoilless y-ray emission spectra and magnetic measurements indicate that ytterbium in YbB4 has an intermediate valence state as in YbAl3... 

By convention, the carbenes displayed in Fig. 1.7 are normal NHCs as they are coordinated to the metal centre throngh the atom. By contrast, abnormal (also named non-classical or unnsnal) are those bound through the atom. Abnormal is also a term used for NHCs having a valence representation requiring additional charges. Remote is a term nsed to describe a carbene which does not have any heteroatom on the a-position to the carbenic carbon (Fig. 1.8) [57]. 

A close analogy to the carbon radicles is provided by certain nitrogen compounds, which are likewise free unsaturated complexes partaking of the character of atoms and having an abnormal valency. With nitrogen, as with carbon, the existence of radicles also depends on the presence of aromatic rings. Tetraphenylhydrazine corresponds to hexa-phenylethane. 

TOTAL PROTON COUNT FOR THIS STRUCTURE IS (P FOR PROGRAM ESTIMATE) P TOTAL PROTON COUNT BASED UPON NORMAL CONDITIONS IS 8 ARE THERE ANY ABNORMAL VALENCE OR CHARGE CONDITIONS WHICH WOULD AFFECT THIS COUNT (Y/N) N PROTON COUNT FOR NODE 2 (D TO DISPLAY STRUCTURE) ... 

Ionic Radii in A of Ions with Abnormal Valencies... 

Some metals crystallize in more than one structural type, which means that there are two alio tropic modifications. The metals marked do not conform precisely to the closest-packed structure, but deviate slightly from it. Uranium, manganese, gallium and indium have very abnormal structures, and the last two are transitional between metallic and non-metallic elements of the carbon group. The picture presented by the metallic structures is utterly different from that of elements of the four last groups of the periodic system. The homopolar bonds of these latter strive to produce a state in which the number of neighbours of each atom is determined by its valency. In the other elements, however, forces appear to be acting that tend to surround each atom with as many other atoms as possible. 

Rule 8-Abnormal valence angles may be found if they increase HOMO-LUMO interactions between molecular fragments... 

In order to circumvent this difficulty, the simplest way is to resort to nomenclature based on pentalene and name structure 10 1,6,6a SIV-trithiapentalene. This name brings out the abnormal valency of the sulfur atom numbered 6a (6aS1V) and permits us to apply the rule stating that such fusion names are related to the structure containing the greatest number of noncumulative double bonds. 

Partial substitution of A and B ions is allowed, yielding a plethora of compounds while preserving the perovskite structure. This brings about deficiencies of cations at the A-or B-sites or of oxygen anions (e.g. defective perovskites). Introduction of abnormal valency causes a change in electric properties, while the presence of oxide ion vacancies increases the mobility of oxide ions and, therefore, the ionic conductivity. Thus, perovskites have found wide apphcation as electronic and catalytic materials. 

According to Hund s rule, electron shells are stable when empty, full or half-full. For example, the configurations 4f° (La +), 4f (Gd +), and 4f (Lu +) are stable. Ce +, Pr +, and Tb " " have one or two more electrons than required for stable electronic configurations so they can be further oxidized to a A+ state. In contrast, Sm +, Eu +, and Yb + have one or two less electrons than required for a stable electronic configuration and they, therefore, tend to receive one or two electrons and undergo a reduction to a 2+ state. These are the reasons for these elements having abnormal valence states. 

Intimately connected to the choice of bending parameters is the choice between a general valence force (GVFF) expression (including a stretch-bend interaction) or a UBFF representation. The necessity for the inclusion of either a stretch-bend or a Urey-Bradley term to account for the abnormally long bonds in e.g. norbomane was pointed out by several authors 17,22,31), Several representations have been proposed normal non-bonded interactions 24) the classical UB expression 21) and whittled atoms 7,31,56) (smaller radii in the direction of the geminal atoms), but a further theoretical analysis of the UB potentials seems mandatory. 

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