Abstract The kinetics of the aquation reaction of trans- chloro-tertiary phosphine-Co III-bisdimethylglyomixato complexes and pKa of the corresponding aquo-complexes have been examined in order to determine the trans-effect of the tertiary phosphine on the chloro and aquo axial ligand respectively. The aquation rates, the pKa of aquo-complexes, and half-wave potentials of reduction of the aquo-complexes at d.m.e. correlated with the pKa of the phosphine. The results are discussed in term of σ-donor character of the phosphine and trasmission of electronic effect through the cobalt atom. The effect of the planar chelating system on the kinetic properties in model molecules of vitamin B 12 is discussed.