Fig. 3. Unequal distribution of bond valences required by the valence-sum rule, shown for a corner-sharing dimer (3 v.u. for N³⁻, green; 5 v.u. for P⁵⁺, orange); even distribution of bond valences (4 × 1.25 v.u.) would leave the bridging ^[2]N³⁻ ion under-bonded by 0.5 v.u.