C.-T. adducts

In this chapter we will not attempt to give an overview of all of the knowledge on the reactivity of chalcogen-donor molecules towards di- and inter-halogens, since some aspects of this are discussed in other chapters. Instead, this chapter is dedicated exclusively to the analysis of the chemical bond and structural features of C.-T. adducts between LE chalcogen-donor molecules (E = S, Se) and di- and inter-halogens, and their reactivity towards metal powders. 

Based on structural, vibrational, and electronic features, C.-T. adducts of S-donor molecules with I2 (the most numerous) were classified in three categories 25 28 (i) Weak or medium weak characterised by a mutual perturbation... 

Most of the reported structurally characterised neutral spoke C.-T. adducts have sulfur as the donor atom and di-iodine as the acceptor molecule.1,2 10 12 13 18,28 38 Those obtained from molecules containing selenium and di-iodine are less numerous,1,2 39 40... 

Under such circumstances, the E-X and X-Y bond distances should be strongly correlated in C.-T. spoke adducts. In fact, it is well known that for C.-T. spoke I2-adducts with S-donors a reciprocal correlation exists between (f(I-I) and rf(S-I), which was initially proposed by F.H. Herbstein and W. Schwotzer as a hyperbola.46 Assuming a valence (bond order) model for the description of the S-I-I system within C.-T. adducts, with n(I-I) + n(E-I) = 1... 

All these extended spoke adducts structurally resemble Z -shaped X82-polyhalides (X = I, Br) in which two asymmetric X3 trihalides are bridged by an X2 molecule.15 This further supports the above-mentioned similarity in the chemical nature of asymmetric X3 trihalides and linear E-X-Y systems in C.-T. adducts. 

A different structural motif is observed in the compound with the stoichiometry (tu)3(I2)5 (tu = thiourea) which according to the authors, consists of a tu-I2 spoke -like strong C.-T. adduct, a (tu)2(I3) + cation counter-balanced by an I3, and a perturbed di-iodine molecule.17 The strong tu-I2 adduct and the V -shaped planar tu-I I- I-tu + cation are bridged at their terminal an central iodide atoms, respectively, by a di-iodine molecule (Figure 9). 

Figure 10 Sulfur-donor molecules whose C.-T. adducts with I2 or IBr have been used as oxidising reagents towards metal powders...

Therefore, other factors that have not yet been studied and are not easily quantifiable, such as the absorption properties of the C.-T. adduct at the surface of the metal powder and the solubility of the formed species should be important in determining the oxidation properties of C.-T. adducts towards metal powders. Furthermore, some extrinsic factors inherent to the experimental conditions, such as reaction temperature, reagent concentration, and nature of the solvent have been reported to affect the overall yield or the course of the reaction, and led to separation of different products in some cases.55 59 In any case, it appears that the simultaneous presence of the donor molecule and the di-/inter-halogen lowers the oxidation potentials of the metals, allowing their oxidation, dissolution, and complexation. 

The C.-T. adduct Me2dazdt 2I2 (Me2dazdt = /V,/V -dimethylperhydrodiaze-pine-2,3-dithione, see Figure 10), which proved to be air-stable, was successfully reacted in THF at room temperature with gold(0)61 and palladium(0)62 in powder, and with liquid mercury 63 conversely, no reactivity of this adduct with platinum(0) and rhodium(O) was observed, even on refluxing the solvent. 

Although the salt tV,7V -dimethylpiperazinium-2,3-dithione triiodide [(Me2pipdt)I3] is not a C.-T. adduct, it is interesting to report its reactivity towards platinum in powder, with formation of the platinum(II) complex [Pt(Me2pipdt)2](I3)2.65... 

With bromine, diorganosulfldes tend to furnish charge transfer adducts, whereas selenides can give equilibria between C.T. adducts and products from oxidative addition, i.e. molecular Se(IV) compounds.23 A particular example of such equilibria is the bromination of selenanthrene reported by Nakanishi et al.24 (see Chapters 8.2 and 10.3).24... 

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