Treatment of 1,2,3,4-tetrahexylbenzene or 1,2,4,5-tetrahexylbenzene with Hg(OOCCF3)(2) produces 1,2-phenylenedimercury bis(trifluoroacetate) 7 or its 1,4-isomer 9, which an bidentate Lewis acids. IR studies in CH2Cl2 established that 1,2-isomer 7 forms doubly coordinated 1:1 complexes with amides in which both atoms of mercury interact simultaneously with the carbonyl oxygen atom of the bound amide, whereas 1,4-isomer 9 forms more weakly bound singly coordinated complexes. Variable-temperature IR studies revealed that for the double coordination of diethylformamide (DEF) by 1,2-isomer 7, Delta H degrees = -4.5 +/- 0.2 kcal mol(-1) and Delta S degrees = -6.9 +/- 1.0 eu, whereas for the single coordination of DEF by 1,4-isomer 9, Delta H degrees = -3.4 +/-0.2 kcal mol(-1) and Delta S degrees = -8.1 +/- 1.0 eu. These measurements establish that double coordination is significantly more exothermic than single coordination. In addition, variable-temperature NMR studies showed that Delta G double dagger for rotation around the N-C(O) bond of dimethylpivalamide (normally 12.2 +/- 0.1 kcal mol(-1) in CDCl3) is increased more by the addition of an equimolar amount of 1,2-isomer 7 (12.9 +/- 0.1 kcal mol(-1)) than by the addition of an equimolar amount of 1,4-isomer 9 (12.3 +/- 0.1 kcal mol(-1)). This demonstrates that the double coordination of carbonyl compounds by bidentate Lewis acids can have measurable effects on the rates of subsequent chemical transformations.