Weak field ligands do not cause much crystal field splitting, so the crystal field splitting energy is lesser than pairing energy ( ∆_o<P ). Since pairing energy is higher than crystal field splitting energy, the electron will prefer not to pair up, therefore they will form paramagnetic(have unpaired electrons) octahedral complexes but for strong-field ligands, pairing of electrons is preferred because pairing energy is lower than crystal field splitting energy(∆_o>P). A route where lesser energy is spent will always be preferred because the complex will be more stable in this case.