Mo ¨ssbauer and Raman spectroscopies, calorimetric measurements, the LIESST effect (light induced excited spin state trapping) and dynamics of the high-spin (HS) to low-spin (LS) relaxation have been investigated in order to thoroughly study the spin-crossover compounds of formula {Fe(L)[H 2 B(pz) 2 ] 2 }, where L ) 2,2′bipyridine (1) and 1,10-phenanthroline (2). The Mo ¨ssbauer studies have revealed the occurrence of an almost complete spin conversion at low temperature, while 9.5% (1) and 15% (2) of molecules remain LS at room temperature. The thermal variation of the proportion of HS molecules, n HS , obtained from the Mo ¨ssbauer spectra has been used to calibrate the thermal dependence of n HS deduced from the magnetic susceptibility measurements. The thermodynamic Slichter and Drickamer model was used to fit the thermal variation of n HS . The estimated thermodynamic parameters are ∆H ) 7.7 kJ mol -1 , ∆S ) 47.4 J mol -1 K -1 and interaction parameter Γ ) 2.1 kJ mol -1 for 1 and ∆H ) 7.9 kJ mol -1 , ∆S ) 48.3 J mol -1 K -1 and Γ ) 3 kJ mol -1 for 2. These data agree with those obtained from heat capacity experiments: ∆H ) 7.7 ( 0.4 kJ mol -1 and ∆S ) 48 ( 3 J mol -1 K -1 for 1, and ∆H ) 8.0 ( 0.4 kJ mol -1 and ∆S ) 49 ( 3 J mol -1 K -1 for 2. The transition temperatures, estimated from the heat capacity peaks, were T c ) 159.5 K for 1, and T c (up) ) 165.0 K, T c (down) ) 162.7 K for 2, in heating and cooling processes. Raman spectra have been recorded for 2 both in the HS (300 K) and LS (110 K) states. Significant changes occur in the position of several low-frequency Raman modes in the low-temperature spectrum. The entropy change associated with the 15 normal vibrational modes of the [FeN 6 ] pseudo-octahedron has been estimated from the measured spectra as ∆S vib ) 36 J K -1 mol -1 , and the total entropy change is ∆S ) ∆S vib + ∆S el ) 49 J K -1 mol -1 . This value is comparable to that obtained from the calorimetric measurements. Photomagnetic studies revealed a complete conversion from the LS into the HS state upon irradiation at 10 K in the 647.1-676.4 nm wavelength region. The critical LIESST temperatures are 52 K (1) and 44 K (2). The dynamics of the LS to HS conversion was studied for the two compounds in the 6-50 K temperature range. In the thermally activated region the apparent activation energy, E a , was estimated to be 240 cm -1 for 1 and 170 cm -1 for 2. In the tunneling region the rate constants were estimated to lie in the 9 × 10 -7 to 1 × 10 -6 s -1 range.