A comparison of DFT-computed and measured infrared spectra reveals the ground state structures of a series of gas-phase silicon clusters containing a common Mn 2 unit. Mn 2 Si 12 and [Mn 2 Si 13 ] + are both axially symmetric, allowing for a clean separation of the vibrational modes into parallel (a 1 ) and perpendicular (e 1 ) components. Information about the Mn–Mn and Mn–Si bonding can be extracted by tracing the evolution of these modes as the cluster increases in size. In [Mn 2 Si 13 ] + , where the antiprismatic core is capped on both hexagonal faces, a relatively simple spectrum emerges that reflects a pseudo-D 6 d geometry. In cases where the cluster is more polar, either because there is no capping atom in the lower face (Mn 2 Si 12 ) or the capping atom is present but displaced off the principal axis (Mn 2 Si 13 ), the spectra include additional features derived from vibrational modes that are forbidden in the parent antiprism.

MC-SCF wavefunctions for three endohedral Mn/Si clusters, Mn2Si10, Mn2Si12, and [Mn2Si13]+, show evidence for strong static correlation, both in the Mn-Si bonds (’in–out correlation’) and between the two Mn centers (’up–down correlation’). We use both Restricted and Generalized Active Spaces (RAS and GAS) to place constraints on the configurations included in the trial wavefunction, showing that, particularly in the high-symmetry cases, the GAS approach captures more of the static correlation. The important correlating pairs are similar across the series, indicating that the electronic structure of the endohedral Mn2 unit is, to a first approximation, independent of the size of the silicon cage in which it is embedded.