Tellurium secondary interactions

The heavier chalcogens are more prone towards secondary interactions than sulfur. In particular, the chemistry of tellurium has numerous examples of intramolecular coordination in derivatives such as diazenes, Schiff bases, pyridines, amines, and carbonylic compounds. The oxidation state of the chalcogen is also influential sulfur(IV) centres engender stronger interactions than sulfur(II). For example, the thiazocine derivative 15.9 displays a S N distance that is markedly longer than that in the corresponding sulfoxide 15.10 (2.97 A V5. 2.75-2.83 A, respectively). ... 

From elemental sulfur to selenium and tellurium, intermolecular interactions (,secondary bonds, soft-soft interactions) play an increasing role. According to N. W. Alcock,1 the term secondary bond describes interatomic distances longer than covalent single bonds but shorter than van der Waals interatomic distances.1 In many cases secondary bonds can also be described as coordinative Lewis base - Lewis acid or charge transfer (donor-acceptor) types of interactions. 

Two examples (from our own work) illustrate intramolecular secondary interactions. Thus, in triphenyltellurium(IV) tetraphenyldithioimidodiphosphinate, Ph3Te [SPh2PNPPh2S] , 27, the tellurium-sulfur interatomic distances, 3.264 and 3.451 A, are significantly longer than the sum of the covalent radii (2.443 A) but significantly shorter than the sum of the van der Waals radii (3.86 A) [56], In bis(diphenyldithiophosphinato)ditellurium(I), Te2(S2PPh2)2, 28, there are two types of tellurium-sulfur interaction - normal covalent, i.e. primary bonds (Te-S 2.471 and 2.493 A) and secondary bonds (Te- -S 2.989 and 3.066 A) [57]. 

As discussed in detail in Section 2.4, the pronounced tendency of tellurium to participate in both hypervalent bonding and secondary 5p2 —> 5a interactions can lead to the construction of polymeric telluride networks. 

There are single examples of sulfur and selenium xanthates with the remaining structures to be described in this section featuring tellurium as the central element. There are a significant number of binary xanthates and, in common with these, their organotellurium xanthates feature extensive supra-molecular association, usually, but not exclusively, via Te- S secondary bond interactions. A rare example of mixed-ligand 1,1-dithiolate structure is available where the odd dithiolate ligand is a dithiocarbamate. 

The propensity of tellurium to form intermolecular interactions distinguishes its chemical properties from those of sulfur and selenium for which such secondary bonding is virtually nonexistent (see Ref 38 and references therein). By contrast to polysulfides and polyseleiudes, polytelluride anions can exhibit charges that deviate from -2. 

However, tellurium commonly shows secondary bonding interactions that expand the coordination environment. It can be seen in Figure 8 that both Me2TeCl2 and (CH2)3TeCl2 3 showtwo close Te- -Cl close contacts of3.412-3.478 A and 3.359-3.479 respectively. The coordination environment around tellurium can therefore be considered as AX4Y2E in which X represents the primary bonding interaction, Y the secondary bonding interaction, and E the tellurium lone-pair. 

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