Experimental and theoretical investigations of the contact Ion pairs formed by reactions of the anions [(EPR2)(2)N](-) (R = Pr-i, Bu-t; E = S, Se) with the cations [(TePR2)(2)N](+) (R = Pr-i, Bu-t)

Reactions of the sodium salts [(EPR2)(2)N]Na(TMEDA) (R = Pr-i, Bu-t; E = S, Se) with the iodide salts [(TePR2)(2)N]l (R = Pr-i, Bu-t) in toluene produce the mixed-chalcogen systems [(EPR2)(2)N][(TePR2)(2)N] (6b, E = Se, R = Bu-t; 6c, E = S, R = Bu-t; 7b, E = Se, R = Pr-i; 7c, E = S, R = Pr-i). Compounds 6b, 6c, 7b, and 7c have been characterized in solution by variable-temperature multinuclear (P-31, Se-77, and Te-125) NMR spectroscopy and in the solid state by single-crystal X-ray crystallography. The structures are comprised of contact ion pairs linked by bonds between Te and S or Se atoms. For the tert-butyl derivatives 6b and 6c, the anionic half of the molecule is coordinated in a bidentate (E,E') fashion to one Te atom of the cationic half to give a spirocycle, whereas in the isopropyl derivatives 7b and 7c, the anion acts as a monodentate ligand with only one E-Te bond and the second S or Se atom pointing away from the cation. A comparison of the chalcogen-chalcogen bond orders in 6b, 6c, and the all-tellurium system 6a (E = Te), as determined from the experimental bond lengths, shows that the Te-Te bond order in the cations decreases as the strength of the E-Te interaction increases. This trend is attributed to increased electron donation from the anion into the lowest unoccupied molecular orbital [sigma*(Te-Te)] of the cation along the series S < Se < Te. A similar trend is observed for the monodentate contact ion pairs 7b and 7c. Density functional theory calculations provided information about the relative energies of bidentate and monodentate contact ion pair structures and the extent of intramolecular electron transfer in these systems.