  A bond-directional anisotropic exchange interaction, called the Kitaev
interaction, is a promising route to realize quantum spin liquids. The Kitaev
interactions were found in Mott insulators with the strong spin-orbit coupling,
in the presence of quantum interference between indirect electron transfers.
Here we theoretically propose a different scenario by introducing a polar
structural asymmetry that unbalances the quantum interference. We show that the
imbalance activates additional exchange processes and gives rise to a dominant
antiferromagnetic Kitaev interaction, in stark contrast to the conventional
ferromagnetic ones. We demonstrate by ab initio calculations that polar Ru
trihalides with multiple anions, $\alpha$-RuH$_{3/2}X_{3/2}$ ($X$=Cl and Br),
exhibit the antiferromagnetic Kitaev interaction whose magnitude is several
times larger compared to existing candidates. Our proposal opens the way for
materializing the Kitaev spin liquids in unexplored parameter regions.
