pKa is the acid dissociation constant or the measure of strength of acid in a solution. This affects the formation of the complexes since the reaction is dynamic meaning reversible. Consider the equation
HL <-> H+ + L- (dissociation of ligand in a solution)
M + L- <-> ML (formation of metal ligand complex)
M = metal
L = Ligand
ML = Metal Ligand complex.
To determine the Keq or the equilibrium constant for the reaction M + L -> ML, in which the ligand loses a proton, the pKa of the protonated ligand must be known.