Effect of ligand constraints on the geometry of two sulfur-bridged binuclear iron(II) complexes. Structures of bis {.mu.-[N,N'-dimethyl-N,N'-bis(.beta.-mercaptoethyl)ethylenediamine]}-diiron(II) and bis{.mu.-[N,N'-dimethyl-N,N'-bis(.beta.-mercaptoethyl)-1,3-propanediamine]}-diiron(II)