Based on HMO theory a model has been developed, in accordance with the participation of d-AOs on phosphorus at the d π- p π-conjugation of two ligands with π-MOs. Specific for this HMO-d π-p π- -model: (1) two differently d-AOs are considered, (2) a special principle of minimisation is employed, (3) the calculation furnishes self-consistent resonance integrals for the three center bonds on phosphorus, and therefore (4) only the Coulomb integral on phosphorus is free to choose. The model permits assertions about the strength and the mechanism of the influence of substituents on a d π-p π three center system. It shows, that for the appearance of a d π-p π- conjugation the energy difference between the highest occupied MOs of the ligands, and between these MOs and the d-AOs is important.