A new analysis of the intra-unit charge polarization and inter-unit donation for the interaction of ligands with metals is presented. The analysis is based on the calculation of SCF wave functions with constraints imposed on the variational space used for the molecular orbitals of metal-ligand clusters. With this approach, various kinds of ligand and metal charge rearrangement can be studied separately and their energetic contribution to the bonding can be determined. For the first time, this contribution for metal to ligand and ligand to metal donation is established. The constrained space orbital variation CSOV approach also clearly shows the similarities and the differences between the bonding of CO and NH3. The results are obtained for Al4CO and Al4NH3 clusters chosen to simulate the adsorption of the ligands at an on-top site of the Al(111) surface. However, the features found for these model systems have significance for the analysis of ligand-metal bonding in general. The analysis gives new estimates for the importance of charge donations which are different from those commonly believed. It also shows new important features of the interaction. One unexpected feature of the metal-NH3 interaction is that the electrostatic attraction of the effective dipole moments of the metal and ligand units makes an important contribution to the bond. Inter-unit donation of charge does not seem to be important for this system. © 1984 American Institute of Physics.