The two lowest single-particle hole states in two vertically coupled quntum dots (CQDs) are investigated by using the six-band KP model. A bonding-antibonding ground-state transition is observed with interdot distance increasing. This result is counterintuitive, for the antibonding molecular ground state has never been observed in natural diatomic molecules. By comparing the wavafunction component of hole, we verify that the reordering of bonding and antibonding orbitals with interdot distance increasing is caused by spin-orbit interaction of holes.