A combined experimental and theoretical study of dinitrosyl iron complexes containing chelating bis(diphenyl)phosphinoX (X = benzene, propane and ethylene): X-ray crystal structures and properties influenced by the presence or absence of π-bonds in chelating ligands.

Recent discoveries involving the roles of nitric oxide in humans have stimulated intense interest in transition metal nitrosyl complexes. A series of dinitrosyl iron complexes with the formula [(DPPX)Fe(NO)], {DPPX = 1,2-bis(diphenylphosphino)benzene (), 1,3-bis(diphenylphosphino)propane (), and cis-1,2-bis(diphenylphosphino)ethylene ()} has been prepared and characterized through a combination of FT-IR, NMR, UV-vis, X-ray crystallography, and electrochemical techniques. Infrared spectroscopy showed NO shifts to the region of 1723 and 1674 cm for complexes and , and 1708 and 1660 cm for , indicating that ligand acts as a stronger σ-donor. The X-ray crystallographic data showed that and possess the rare conformation while has the conformation. CV studies on compounds , and display two quasi-reversible oxidations with the E° values at 0.101 and 0.186 V, 0.121 and 0.184 V, and 0.019 and 0.342 V, respectively. The larger ΔE value for compound compared with that of and is attributed to the lack of π-bonds between the two phosphorus atoms. Theoretical calculations using density functional theory were carried out on the synthesized compounds and model compounds and the results are consistent with the experimental data. The calculated HOMO-LUMO gaps for compounds and are 3.736, 4.060, and 3.669 eV, respectively, which supports the stronger back-donation for compound than that of compounds and .