Temperature-dependent laser flash photolysis experiments on the low-spin iron(II) systems [M_1−xFe_x(bpy)₃](PF₆)₂ (M=Cd, Mn and Zn,x≈0.01, bpy=2,2′-bipyridine) under external pressure are presented. Below 50 K the high-spin→low-spin relaxation is an almost 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 tunnelling rate constant by two orders of magnitude. An external pressure of 1 kbar accelerates the low-temperature tunnelling process by a factor of 2. [Mn_1−xFe_x(bpy)₃](PF₆)₂ and [Zn_1−xFe_x(bpy)₃](PF₆)₂show a phase transition at ≈1.1 kbar, which increases the tunnelling rate by a factor of about 6.

The thermal spin transition in the diluted mixed crystal [Zn_1−xFe_x(6-mepy)₃tren](PF₆)₂ (x = 0.00025, (6-mepy)₃tren = tris{4-[(6-methyl)-2-pyridyl]-3-aza-3-butenyl}amine) is studied at 1 bar and 1 kbar by temperature-dependent absorption spectroscopy. From thermodynamic analysis of the high-spin (HS) fractions, values for ΔH_HL⁰ and ΔS_HL⁰ of 1551(50) cm⁻¹ and 7.5(5) cm⁻¹/K and a molecular volume of reaction, ΔV_HL⁰, of 22(2) ųresult. Reconsideration of the cooperative effects in the neat [Fe(6-mepy)₃tren](PF₆)₂from Adler et al. [Hyperfine Interact. 47, 343 (1989)] result in a lattice shift, Δ, of 208(15) cm⁻¹ and an interaction constant, Γ, of 109(15) cm⁻¹. Temperature-dependent laser flash photolysis experiments in the spin-crossover system [Zn_1−xFe_x(6-mepy)₃tren](PF₆)₂ and the LS system [Zn_1−xFe_x(py)₃tren](PF₆)₂ in the pressure range between 1 bar and 1 kbar are presented. Above ≈100 K the HS→LS (low-spin) relaxations behave classically, whereas they become almost temperature independent below 50 K. At ambient pressure, the low-temperature tunneling rate constant in[Zn_1−xFe_x(py)₃tren](PF₆)₂ is more than three orders of magnitude larger than the one in[Zn_1−xFe_x(6-mepy)₃tren](PF₆)₂. External pressure of 27 kbar accelerates the low-temperature tunneling process by almost nine orders of magnitude. The kinetic results are discussed within the theory of nonadiabatic multiphonon relaxation.

Laser flash photolysis experiments were performed on the mixed crystal [Zn_1−xFe_x(6-mepy)₃tren](PF₆)₂ (x=0.00025) at 10 K in the pressure range between 1 bar and 20 kbar. An external pressure of 20 kbar accelerates the low-temperature tunneling process by almost eight orders of magnitude.