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The current efforts to find the materials hosting Kitaev model physics havebeen focused on Mott insulators of d^5 pseudospin-1/2 ions Ir^{4+} and Ru^{3+}with t_{2g}^5(S=1/2, L=1) electronic configuration. Here we propose that theKitaev model can be realized in materials based on d^7 ions witht_{2g}^5e_g^2(S=3/2, L=1) configuration such as Co^{2+}, which also host thepseudospin-1/2 magnetism. Considering possible exchange processes, we havederived the d^7 pseudospin-1/2 interactions in 90^{\circ} bonding geometry. Theobtained Hamiltonian comprises the bond-directional Kitaev K and isotropicHeisenberg J interactions as in the case of d^5 ions. However, we find that thepresence of additional, spin-active e_g electrons radically changes the balancebetween Kitaev and Heisenberg couplings. Most remarkably, we show that theexchange processes involving e_g spins are highly sensitive to whether thesystem is in Mott (U<\Delta) or charge-transfer (U>\Delta) insulating regime.In the latter case, to which many cobalt compounds do actually belong, theantiferromagnetic Heisenberg coupling J is strongly suppressed and spin-liquidphase can be stabilized. The results suggest cobalt-based materials aspromising candidates for the realization of the Kitaev model.

Pseudospin exchange interactions in d^7 cobalt compounds: Possible realization of the Kitaev model