Structures and spectra of bis-tripodal iron(II) chelates, [FeL2](2+), where L= tris(pyrazol-1-yl)methane, tris(pyridin-2-yl)methane, bis(pyrazol-1-yl)(pyridin-2-yl)methane and tris(pyridin-2-yl)-phosphine oxide. Magnetism and spin crossover in the (pz)(3)CH case

A range of six-co-ordinate bis-tripodal iron(II) chelates, [FeL2](2+), has been synthesized from iron-(II) or -(III) precursors where L = tris(pyrazol -1-yl)methane (pz)(3)CH, tris(pyridyl-2-yl)methane (py)(3)CH, bis(pyrazol-1 -yl)(pyridin-2-yl)methane (pz)(2)(py)CH and tris(pyridin-2-yl)phosphine oxi de (py)(3)P=O. Crystal structures have been determined for three compounds and in the (pz)(3)CH case for two polymorphs (alpha and beta) which were ob tained in two different laboratories from iron(III) salts using different c rystallisation solvents. The electronic properties of the [FeL2](2+) series have been investigated by optical spectroscopic measurements. Low-spin d(6 ) behaviour is indicated. Magnetic and Mossbauer effect measurements on [Fe {(pz)(3)CH}(2)](2+) show that a gradual spin crossover to the high-spin sta te occurs above 270 K, though this is far from developed at 350 K. Comparis ons are made with the well studied boron ligand analogue [Fe{(pz)(3)BH}(2)] , which displays similar crossover behaviour, and with bis(tridentate) liga nd complexes containing the ligand 1,4,7-triazacyclononane.