Thallium-oxygen bonds

Secondary Tl- -O bonds can connect polymeric chains of organothallium derivatives into ribbons extended along one of the crystal axes. Thus, dimethyl-thallium(III) rodaninate, 116 [341], and dimethylthallium(III) (r-phenylalaninate), 117 [342], which are themselves polymers based upon thallium-oxygen bonds from the ligand to the metal, are interconnected in double chains by weaker (and longer)... 

I,6-Dioxa-6a-thiapentalene (4).5 Addition of thallium(IU) trifluoroacetate in acetonitrile to a solution of 4//-pyrane-4-thione (1) in acetonitrile at room temperature gives the labile pyrylium salt (2), which on addition of water gives 1,6-dioxa-6a-thiapentalene (4) by sulfur-oxygen bond formation. The intermediate (3) is suggested. 

Boyd et al. [46] have studied the y-radiolysis of several oxyhalides. The extent of reaction in the Group lA and thallium bromates increased linearly with dose, yielding BrOj, BrO", Br and in proportions which depended on the cation present. Later work extended the reactants to KCCOj (products CCO, CCO jUnd O ), CsBrO 3 (products BrO" and O3 ) and CSIO3 (increased absorption was ascribed to molecular iodine in the crystal). It was concluded that reaction within the crystal is initiated by ionization, with excitation of the oxyhalide ion, followed by rupture of the halogen-oxygen bond ... 

Few, mostly thallium-oxygen, compounds are mentioned here, because most organothallium derivatives self-assemble via weaker bonds which can be properly regarded as secondaiy bonds in some borderline cases differentiation is difficult. 

Supramolecular self-organization in the solid state has been observed (by X-ray diffraction) for several thallium-oxygen compounds. The secondary Tl - O bonds in these compounds are rather strong and it is debatable whether these should be regarded as electron-pair donor-acceptor bonds or as secondary bonds. 

Polymeric ribbons of self-assembled chelate molecules, with participation of dative thallium-oxygen and secondary thallium-sulfur intermolecular bonds are present in dimethylthallium(III) diethylmonothiocarbamate, Me2Tl[S(0)CNEt2], 49 [170], and in diphenylthallium(III) diphenylmonothiophosphinate, Ph2Tl[S(0)-PPh2], 50 [171]. 

Taylor, E.C., Altland, H.W., Kienzle, F., and McKillop, A., Thallium in organic synthesis. XLl. Synthesis of 1-substituted 2(lH)-pyridones. New synthesis of unsymmetrical biphenyls via photochemical nitrogen-oxygen bond cleavage of l-aroyloxy-2(liJ)-pyridones, /. Org. Chem., 41, 24-27, 1976. 

The repulsion of the thallium and oxygen lone pairs lead to a distortion of the Tl-6s lone pair, which is no longer totally spherical as one would intuitively expect for an s orbital. The thallium contributions involved in the anti-bonding combination are indeed of 97.7% s-, 1.8% p- and 0.5% d-character. Thus, a much less than expected p-character is found for the thallium orbital, which shows that extensive s-p mixing or hybridization is not essential for the lone pair to become stereochemically active. 

Among some metal oxygen compounds which add, palladium and thallium ion both oxidize olefins and apparently the initial step is the addition of a metal hydroxide across the olefin double bond. The intermediates have not been isolated because they go on to other products but kinetic and other evidence indicates that the addition of the hydroxide is the initial step. In the well known mercury acetate addition to olefins in alcohol solution one can isolate the /S-hydroxv or alkoxy ethylmercury derivatives. 

More involved studies of the oxidation of plant phenols [27], as well as the introduction of thallium and hypervalent iodine complexes and the use of electrochemical methods, have emphasized the importance of another intermediate involved in oxidative coupling reactions, namely the phenoxonium ion 8 [28-30]. Due to its ionic nature, reaction through an oxo-nium ion can improve the regioselectivity of bond formation and lead to fewer unwanted products (for example, no coupling via the oxygen atom). The coupling reaction can then be viewed as an electrophilic aromatic substitution between 17 and a nucleophilic aromatic unit 15 (Scheme 5). 

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