  The exchange interaction between 5d electrons in t$_{2g}$ orbitals is derived
in the J-J coupling scheme which is the appropriate basis in the case of strong
spin-orbit interaction. From simple calculations, it is found that
ferromagnetic Ising interaction (quantum compass model) occurs due to a
selection rule of hybridization between $\Gamma_7$ and $\Gamma_8$ orbitals,
while antiferromagnetic Heisenberg interaction appears from hybridization
between $\Gamma_7$ orbitals. It is also found that the ferromagnetic Ising
interaction becomes small as the spin-orbit interaction increases. Thus, the
sign of exchange interaction changes from ferromagnetic Ising to
antiferromagnetic Heisenberg interaction as the spin-orbit interaction
increases.
