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57Fe Mössbauer data are presented to show that polycrystalline samples of [Fe(dppen)2X2]·2S, where dppen is cis-1,2-bis(diphenylphosphino)ethylene and X- is Cl- or Br- and S is CHCl3 or CH2Cl2, exhibit the light-induced excited spin state trapping (LIESST) effect. If the sample is kept at 10 K, Ar ion laser light converts the whole sample from the thermally stable low-spin (LS) form to the metastable high-spin (HS) form, while light of λ > 695 nm converts some of the metastable HS to the stable LS form. The relaxation rate was monitored in the 28−70 K range following the LIESST effect at 10 K. The plots of the fraction of HS complex (γHS) vs time can be fitted reasonably well to a single exponential for the four complexes. The relaxation data were also analyzed with a model for cooperative (γHS-dependent decay) relaxation. It is found that complex 1 shows less cooperativity than the other three complexes. Furthermore, the HS → LS relaxation times observed in the ∼28−70 K range for these four complexes are relatively long compared with those for FeN6 complexes under similar conditions. The relaxation kinetics of [Fe(dppen)2Cl2]·nCHCl3 (1) appears to be affected by the amount (n) of solvent molecules in the crystal lattice. Variable-temperature magnetic susceptibility data show that only when each complex has fully two solvent molecules does the conversion from HS at high temperatures to LS at low temperatures go to completion. The results of the X-ray structures of [Fe(dppen)2Br2]·2CHCl3 (3) at 149 and 193 K are given, i.e., below and above the LS to HS (T 1/2 = 175 K) conversion. At both temperatures, complex 3 has the monoclinic space group P21/c, which at 149 K has a unit cell with a = 11.494(16) Å, b = 12.895(14) Å, c = 17.49(2) Å, and Z = 2. Refinement of the 149 K data set with 3434 observed [F o > 4σ(F o)] reflections gave R = 0.0816 and R w = 0.1014. There is a large increase in the average Fe−P bond length of 0.27 Å from 149 to 193 K, whereas the Fe−Br bonds only increase by 0.033 Å. The relatively large change in Fe−P bond lengths must be largely responsible for the slow rate of tunneling from the metastable HS state to the stable LS state.

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