Intermolecular Te- • ■ N contacts

The molecular and crystal stmctures of 2-phenylbenzotellurazole were determined by X-ray (89KGS1690). The dihedral angle between the planes of the five-membered heterocycle and 2-phenyl ring is 31.2°. In contrast to benzoisotel-lurazole, no shortened intermolecular Te- N contacts were found in the crystal stmcture of 2-phenylbenzotellurazole. Consequently, no anomalies in solubility and melting point were revealed for this compound as compared with its sulfur and selenium congeners. 

Te—N bonds also consist of quasi-polymeric structures. Thus benzoisotellurazole (13) shows short intermolecular Te N contacts (2.4 A) <78JHC745>. Intermolecular Se N bonds are also present in 1,2,5-selenadiazoles but they are weaker and do not give rise to polymeric structures <84AX(C)653>. 

The role of intramolecular n - ct donor-acceptor interactions in compounds TeCl2(OR)2 and TeCl3OR (R=alkyl with B-donor atom) has been elucidated experimentally and with help from ab initio calculations.110 Solid (Me0CH2CH20)2TeCl2 exhibits a network of intramolecular O- -Te and intermolecular O- -Te (267.4 pm) and Cl- -Te (357.7 pm) contacts involving heptacoordinated tellurium. 

However, because of the weakness and nonspecific nature of vdW forces, nonbonded distances vary more widely than bond lengths. By a statistical analysis of numerous structures, Rowland and Taylor derived a novel system of vdW radii (Figure 3, Table 3). Atoms of Cl, Br, I, S, Se and Te, forming one covalent bond, show a highly anisotropic shape in intermolecular contacts (Figure 4), which can be described as an ellipsoid compressed along the bond direction, while harder N, O and F atoms remain essentially spherical. 

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