Though protein embedded Chlorophylls almost exclusively exhibits five-coordination of the Mg atom, six-coordination has been well established in some solvents. The factors governing the coordination number are uncertain since insufficient thermodynamic data are available. To investigate this biologically important question, we report calculations on mono and diaxial coordination of pyridine and water to Mg-porphyrin using MP2 and density functional (corrected for dispersion) methods. In agreement with experiments, binding of a second axial ligand is exergonic for pyridine but not for water; the large value of the dispersion energy in the first case is found to be the key factor. (C) 2009 Elsevier B.V. All rights reserved.