Tellurium complexes four-coordinate

Several four-coordinate selenium(U) complexes with bidentate ligands, all of which involve highly distorted square planar SeS4 or SeSe4 coordination spheres, and some tellurium analogues have been described.11... 

Square-planar, four-coordinate tellurium(II) complexes, whose structural chemistry has recently been summarized by Foss 89>, offer illustrations of this highly approximate, constant-volume rule. Fig. 27 is a drawing, approximately to scale, of a schematic, electron-domain representation of a section through the 1 1 complex of benzenetellurenyl chloride with thiourea 90>. Shown are the electron domains of the tellurium kernel (largest, nearly centrally located solid circle) the ligands kernels (two chlorine kernels, a sulfur kernel of thiourea, and a carbon kernel of benzene) and the domains of the tellurium atom s shared valence-shell electrons (open circles) and, most schematically of all, the tellurium atom s unshared valence-shell electrons (shaded region). 

In contrast to alkali metals or transition metals, complexation of [R2P(E)NP(E )PR 2] with main group elements is less well documented. Syntheses and structures of a number of tellurium complexes Te[R2P(E)N-P(E )PR 2]2 (E = S, Se) have been reported. It has been argued that electronic rather than steric effects control the geometry around the central tellurium atom. As examples four-coordination in (79) and two-coordination in (80). ... 

A large number of tellurium(II)-complexes exist also with an electron-rich three-center bond 12 7). In most complexes the tellurium(II) shows squeire planar four coordination. 

The structure of 169, although not crystallographically characterized, is assumed to be as drawn in Scheme 2 with bent, two-coordinate tellurium atoms, but photolysis or thermolysis converts this complex into the isomeric species 168, which contains a Te2 ligand acting as a four-electron donor. If 168 is assumed to contain a doubly bonded ditellurium moiety, Te=Te, electron donation occurs from the n bond and one Te lone pair, as illustrated in E. 

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