MATRIX EFFECTS ON THE HIGH-SPIN-]LOW-SPIN RELAXATION IN [M1-XFEX(BPY)(3)](PF6)(2) (M = CD, MN AND ZN, BPY = 2,2'-BIPYRIDINE)

Temperature-dependent laser flash photolysis experiments on the low-sp in iron(II) systems [M1-xFex(bpy)(3)](PF6)(2) (M = Cd, Mn and Zn, x ap proximate to 0.01, bpy = 2,2'-bipyridine) under external pressure are presented. Below 50 K the high-spin --> low-spin relaxation is an almo st temperature-independent tunnelling process. Above that temperature it tends towards a thermally activated behaviour. A change of the host from cadmium to zinc results in an increase of the low-temperature tu nnelling rate constant by two orders of magnitude. An external pressur e of 1 kbar accelerates the low-temperature tunnelling process by a fa ctor of 2. [Mn1-xFex(bpy)(3)](PF6)(2) and [Zn1-xFex(bpy)(3)](PF6)(2) s how a phase transition at approximate to 1.1 kbar, which increases the tunnelling rate by a factor of about 6. (C) 1998 Elsevier Science B.V . All rights reserved.