Back-bonding signature with high pressure: Raman studies on silver nitroprusside

Abstract

In centrosymmetric molecules, like An+[M(CN)6]n− (where A is alkali metal cation), normally all stretching vibrations of cyanide (CN–) shift to high frequency in response to nonhydrostatic pressure, whereas, in non-centrosymmetric molecules in which one axial CN ligand is replaced by NO ligand, one observes unusual softening of only equatorial CN stretching modes. This effect is pronounced when A+ is replaced by Ag+ with difference in coordination ability of latter, resulting in expression of characteristic signature of back-bonding. One can correlate this uneven stretching of cyanide to Poisson-like effect, where the axial Fe–N, Fe–C, and C–N stretching modes harden but the equatorial C–N stretching modes soften due to expansion at the equatorial plane. Thus, the present study is focused on results of non-hydrostatic high-pressure Raman measurements on silver nitroprusside up to 11.5 GPa, for not only observing characteristic signature of “back-bonding” interaction, rarely featured in literature, but also for generating reversible flexible structures akin to noncovalent interaction.