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)3CH case