| Abstract: | The vibrational properties of the two octahedral FeII dinitrogen complexes [FeH(N2)(depe)2]+ ( 1 ) and [FeCl(N2)(depe)2]+ ( 2 , depe = 1, 2‐bis(diethylphosphino)ethane) are investigated with the help of infrared and Raman spectroscopies. Vibrational data are evaluated with a Quantum Chemistry Assisted Normal Coordinate Analysis (QCA‐NCA; N. Lehnert, F. Tuczek, Inorg. Chem. 1999 , 38, 1659). In agreement with high values found for ν(NN) and the corresponding force constants f(NN), the N2 ligands in compounds 1 and 2 are non‐activated which corresponds to the observation that N2 is not protonable in FeII systems. Taking into account the short Fe‐N bond lengths, the values of the Fe‐N stretching force constants (2.55mdyn/Å for 1 and 2.58mdyn/Å for 2 ) are found to be compatible with those of other FeII low‐spin compounds coordinated to backbonding N‐coordinating ligands. The force fields obtained for the Fe‐N2 units of 1 and 2 are almost identical although the thermal stability of 1 and 2 with respect to loss of N2 is different. This indicates that the zero‐point vibrational levels are unaffected by possible ground‐state level crossing processes occuring at larger Fe‐N bond lengths, as observed for 2 (O. Franke, B. E. Wiesler, N. Lehnert, C. Näther, V. Ksenofontov, J. Neuhausen, F. Tuczek, Inorg. Chem. 2002 , 41, 3491). |
