Pressure-dependent $\nu$(CC) and $\nu$(CN) Raman modes of the molecule-based magnets M[TCNE](NCMe)$_2$X

M[TCNE] (M=V, Fe, Mn, Co, Ni; TCNE=tetracyanoethylene) molecule-based magnets demonstrate high magnetic ordering due to a strong antiferromagnetic interaction between the unpaired d- and \mbox{p-electrons } of the metal ions and the ligands; however, the type of bonding involved in the superexchange mechanism remains unclear. The Raman active C=C vibration depends solely upon the degree of charge transfer from the metal ion to the $\pi$* antibonding orbital of the ligand, therefore the strength of the vibration is only sensitive to backbonding. Raman spectra of the M[TCNE](NCMe)$_2$X (M=Fe, Mn, Ni; X=FeCl$_4$, SbF$_6$) molecule-based magnets were collected in a diamond anvil cell at pressures up to 36 kbar. The observed pressure-induced strengthening of the $\nu$(CC) and $\nu$(CN) Raman modes provides a clearer picture of the type and degree of backbonding, which will ultimately help build a model of how superexchange is occurring in these systems.