Nature of the observed ν(NO) band shift and splitting during the 3D to 2D structural change in transition metal nitroprussides

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The electronic structure and related optical properties of the nitroprusside ion, [Fe(CN)5NO]2-, are dominated by the nitrosyl (NO+) group. The light-induced 2b2(xy) → 7e(π*NO) (Fe → NO) and related appearance of meta-stable isomers for the NO group supports many of the functional properties of metal nitroprussides. In recent studies on the preparation of 2D transition metal nitroprussides from their 3D analogs, we have observed that such a structural change is concomitant with a decrease for the ν(NO) frequency and certain band splitting (unfolding). The 3D to 2D structural modification leads to an increase in the electron density at the iron atom. This results in an enhancement for the π*-back donation, which increases the electron population at the π*2px and π*2py orbitals. These last orbitals are doubly degenerated, and the increase for the π*-back bonding induces the removal of that degeneration via the Jahn-Teller effect. This intuitive mechanism explains the mentioned band unfolding related to the structural change. Such explanation was herein supported by a model based on the normal modes for the FeNO fragment derived from the small oscillation formalism around the equilibrium positions for the involved atoms. This model shows that there is a very close relationship between the deformation of the nitrosyl group (NO+) and the splitting of the IR bands. That is, the less linear the nitrosyl group, the greater the shift and splitting of the mentioned IR band. An excellent coincidence between the frequency values calculated using the model and the experimental ones was observed. The largest difference between the theoretically calculated ν(NO) frequencies and the experimental ones was only 6 cm−1 and 5 cm−1 in representative samples of 2D and 3D transition metal nitroprussides, respectively. The understanding of such an effect could be relevant for the functional properties of these 2D coordination polymers.