PARTIAL MOSSBAUER QUADRUPOLE SPLITTINGS IN LOW-SPIN IRON(II) COMPOUNDS - PROPERTIES OF BIDENTATE PHOSPHINE-LIGANDS AND POSSIBILITIES TO FORECAST WHEN COMPLEXES CAN BE PREPARED FOR OCTAHEDRAL LOW-SPIN IRON(II)

The Mossbauer spectroscopic quadrupole splittings (QS) for a series of six-coordinate low-spin iron(II) complexes containing bidentate phosphine ligands are discussed. Partial quadrupole splittings (p.q.s.) are used to assign cis and trans geometries to several of the complexes. From the invariance of the p.q.s. value for Et2PCH2CH2PEt2 (depe) it is suggested that this ligand does not vary its bonding to low-spin iron(II) irrespective of the other monodentate ligands present. The p.q.s. values for the bidentate phosphine ligands are refined and ordered. A correlation between small p.q.s. values and large cone angles is found and put into context. The bidentate phosphine ligands have bonding properties (both electronic and steric) that lie between those of triorganic phosphines and triorganic phosphites of similar sterochemistry around the phosphorus atoms. The range of QS values in the low-spin complexes is found to span from -1.84 to +1.8 mm s-1. Some hypothetical complexes whose predicted QS lies within the QS range and which should not be unstable on steric grounds are listed.