Abstract
Solutions of octahedral adducts SnX 4·2L (X = Cl, Br) in inert solvents in general undergo two exchange processes observable by 1H NMR. The fastest corresponds to neutral ligand exchange on the cis isomer, and the slowest is believed to be the same exchange on the trans adduct. i-SnX 4·2L + *L ⇆ i-SnX 4·L*L + L (i = cis, trans) For the slower process, a cis trans isomerization could not be excluded. Both exchange reactions obey a D mechanism and are first order in adduct with the following activation parameters ΔH* (kcal mol −1) and ΔS* (cal K −1 mol −1): 12.3, +21 ( cis-Me 2CO); 15.3, +24 and 17.8, +15 ( cis- and transMe 2S); 15, +9 and 19, +9 ( cis- and trans-TMPA). The ΔG*'s for the cis and trans adducts differ by 4 to 5 kcal mol −1 at 250 K and increase in the same order as the stability constant sequence: Me 2CO<Me 2O⪡Me 2S<Me 2Se⪡ TMPA⪡HAMPA Reaction rates of the tetrabromide are greater than those of the tetrachloride adducts due to a change in the effective charge on the metal and to steric hindrance.
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