The synthesis and crystallographic characterization of a series of diiron(II) complexes with a sterically hindered bridging phthalazine ligand are presented. The carboxylate-bridged compounds [Fe 2 (m-O 2 CR) 2 (Ph 4 bdptz)] 2' (R0/CH 3 (3); C 2 H 5 (4); CH 2 Ph (5); t-C 4 H 9 (6)), where Ph 4 bdptz is the ligand 1,4-bis[bis(6-phenyl-2-pyridyl)methyl]phthalazine, and the phosphodiester-bridged complex [Fe 2 (m-O 2 P(OPh) 2) 2 (Ph 4 bdptz)] 2' (7) were prepared as small molecule models of the catalytic sites in non-heme carboxylate-bridged diiron enzymes. The phenyl rings of Ph 4 bdptz form a hydrophobic size-constrained pocket in which additional ligands can be accommodated, and they block the possible formation of tetranuclear species. As the steric bulk of the ancillary ligands is increased, the carboxylates shift from a syn , anti to a syn , syn coordination mode. The Mö ssbauer spectra of the diiron(II) compounds clearly reflect the symmetry of the iron coordination environment. # 2002 Elsevier Science B.V. All rights reserved.