P-substituted dibenzo[c.e][1,2]oxaphosphorines and their Pt-complexes are described. In the first place, aryl-dibenzooxaphosporines were synthesized and converted to the corresponding platinum complexes containing only one P-heterocyclic unit. Then P-alkoxy and P-amino dibenzooxaphosphorines including optically active derivatives were prepared and transformed to the corresponding platinum complexes that contained two heterocyclic P-ligands in position cis. Ring opening reaction accompanying the substitution of the starting chloro-dibenzooxaphosphorine furnished 1-hydroxy-1‘-diphenylphosphinobiphenyl that on reaction with the platinum precursor gave a bis(triarylphosphino) complex. The hydroxy-diphenylphosphinobiphenyl was modified by phosphorylation to furnish a special bidentate P-ligand that can be the starting material of a ring Pt-complex. To synthesize additional bidentate P-ligands, methylaminoethanol and L-ephedrine were reacted with two equivalents of chloro-dibenzooxaphosphorine. The resulting bis(dibenzo- oxaphosphorines) were utilized in the preparation of 7-ring platinum complexes.