Polytellurides metal

Figure 16.10 Structures of some metal-polytelluride complexes.

Polychalcogenides are less stable than polysulfides (p. 681). Reaction of alkali metals with Se in liquid ammonia affords M2Se2, M2Se3 and M2Se4, and analogous polytellurides have also been reported (see preceding section). However many of these compounds are rather unstable thermally and tend to be oxidized in air. 

All the dialkali monosulfides are soluble in water and give alkaline solutions. The tellurides are instantly decomposed by air. They are soluble in water, but the solutions are easily oxidized to red polytellurides. The alkali metal tellurides are strong reducing agents which reduce tellurites to metallic tellurium. 

The chemistry of polyselenides, polytellurides, and their metal complexes is very well established. Typical structures of polyselenide dianions are shown in Fig. 16.8.5. In these species, the Se-Se bond distances vary from 227 to 236 pm, and the bond angles from 103° to 110°. The tethered monocyclic structure of Se - in the complex Sr(15-C5)2(Se9) has a three-connected Se atom forming two long and one normal Se-Se bonds at 295, 247, and 231 pm (anticlockwise in Fig. 16.8.5, with the longest bond represented by a broken line). The other Se-Se bonds are in the range 227-39 pm. 

On the other hand, polytellurides only seem to oxidize metals to the +1 or +11 state. Reaction of equimolar amounts of Te4 with M(CO)6 results in disubstitution of CO forming a cu-complex (CO)4MTe4 (M = Cr (45), W (47)47). If an excess of metal carbonyl is used in the presence of poly-telluride anion, multinuclear products can be isolated and metal-metal bonds can also form, leading to clusters. Careful manipulation of reaction conditions and choice of the polychalcogenide anion used makes possible partial oxidation of the metal centers and cluster formation. The reaction of iron carbonyls with polytelluride anions can lead to a wide array of cluster compounds, the identities of which are controlled by the stoichiometries and compositions of the starting telluride anions. For instance, reaction of [Fe(CO)5] with Te2 leads to the formation of [Fe3(CO)9(ju.3-Te)]2 (48),48 whereas its reaction with increasing amounts... 

That the nature of the polytelluride solution plays an important role is also borne out by studies of the chromium-telluride system. If the polytel-lurides are prepared in situ by the reaction of elemental potassium with elemental tellurium in DMF, the products are considerably cleaner and can be obtained in higher yields than when the polytelluride source is a premade potassium salt, such as K2Te2 or K2Te3, It has been suggested that equilibria are occurring in the reaction flask. Pure Te2- and Te2- can be prepared in pure form in liquid ammonia, but there is no evidence for the formation of higher tellurides in the presence of alkali metal counter-... 

Polytellurides, Te , - re less straightforward and often form complex units coordinated to metal centres. The isolated ions Te2 and... 

Chapter 15 Metal sulfides, polysulfides, polyselenides and polytellurides 447... 

Although Se and Te analogues of polysulfanes do not extend beyond the poorly characterized H2Se2 and H2Te2, the chemistries of polyselenides, polytellurides and their metal complexes are well established. Equations 15.43-15.46 illustrate preparations of salts of [Se ] and [Te ] see Section 10.8 for details of crown ethers and cryptands. 

In contrast to the polysulfides and almost all the polyselenides (exception Cs4Sei6 ), a number of polytelluride anions are also known with total negative charges greater than 2, as, for instance, in NaTe with discrete [Tcs]" ions and in 83X022 with polymeric sheet-Uke [Tcg ] ions. These are examples for the striking variety of structural motifs and formula types (x/y = 5/3, 1/1, 2/3, 2/5, 1/3, 1/4, 1/6, 2/13, 1/7 and 3/22) that are known for the alkali metal polytellurides A TCj, and which reflect the propensity of tellurium both to catenate and to participate in hypervalent coordination polyhedra. Whereas discrete anions are observed for xjy > 2/3, polymeric chain and lamellar anionic networks are present in such tellurides when xfy < 2/5. ... 

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