Based on the bond-number equality concept an equation is derived for anion complexes of normal valence compounds with triangularly and/or tetrahedrally coordinated central atoms and anions having one, two, three and four bonds to central atoms: %delta = 4 - (n/m') x [2 - %A^[1] + %A^[3] + 2 x %A^[4]]. %Delta is the ratio of the number of central atoms with triangular anion coordination to the sum of all central atoms in the anion complex. n/m' is the ratio of the number of all A anions to the number of all central atoms C' in the anion complex. %A^[1] is the ratio of the number of anions with one bond to a central atom to the sum of all anions in the anion complex. %A^[3] and %A^[4] are defined accordingly. The equation can be used to formulate the possible crystal chemical formulae, which are characterized by partitions of central atoms and anions according to their bond numbers. Nitridosilicates and selected oxoborates are treated as examples of applications of the equation.