Abstract
A method to visualize the equilibrium between molecular complexes (MCs) and trigonal bipyramidal (TB) adducts for ArSeBr~2~Ar′ has been devised. The TB formation causes large downfield shifts for the ipso carbons δ(C~i~), whereas the upfield shifts are characteristic for δ(C~i~) in the MC formation. Therefore, the equilibrium is clearly proved if such an ArSeAr′ is prepared that shows an upfield shift at one of ipso carbons, whereas the other goes downfield, when bromine is added to the ArSeAr′ solution. The halogen‐induced chemical shifts (Δδ) are measured for 4‐YC~6~H~4~SePh (1a–1g), 2‐YC~6~H~4~SePh (2d, 2e, 2h), 2‐YC~6~H~4~SeC~6~H~4~Br‐p (3d, 3e, 3h), 2‐O~2~NC~6~H~4~SeC~6~H~4~Y‐p (4a, 4e, 4i), and 2,4‐(O~2~N)~2~C~6~H~3~SeC~6~H~4~Y‐p (5a, 5e, 5i), where Y = H (a), OMe (b), Me (c), Cl (d), Br (e), COOEt (f), NO~2~ (g), I (h), and t‐Bu (i). Positive values (downfield shifts) are observed for both δ(C~i~) in 2 and 3, positive and negative values are recorded for each δ(C~i~) in 4, and negative values are detected for both δ(C~i~) in 5. The equilibrium between MC's and TB adducts is clearly shown in 4. The equilibrium constants K (K = [MC]/[TB]) are also calculated: they are ca. 0.20 for 2 and 3 and 0.16, 0.33, and 0.73 for 4i, 4a, and 4e, respectively. The contribution of MC is negligible in 1a, and the contribution of the TB adduct is negligible in 5. The results show that TB adducts become unstable relative to MCs, when the effective electronegativity of the selenium atom and/or the bulkiness around the atom in a selenide are increased. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:369–379, 2001