Abstract
The Cs~3~Mo~6~I^i^~6~I^i^~2−x~Se^i^~x~I^a^~6~ series has been obtained by a solid‐state route. There is evidence for a solid solution between the compositions Cs~3~Mo~6~I^i^~6~I^i^~0.8~Se^i^~1.2~I^a^~6~ and Cs~3~Mo~6~I^i^~6~I^i^~0.4~Se^i^~1.6~I^a^~6~ (space group: R$\bar 3$c, Z=6; a=16.7065(4), __c=__20.5523(4) Å, V=4967.8(2) Å^3^ and a=16.6354(3), __c=__20.5444(4) Å, V=4923.7(2) Å^3^, respectively). The structure of this new series is based on magnetic [Mo~6~I^i^~6~Se^i^~2~I^a^~6~]^3−^ and diamagnetic [Mo~6~I^i^~7~Se^i^I^a^~6~]^3−^ units with 23 and 24 valence electrons per Mo~6~ cluster, respectively. For a particular x, the structure of Cs~3~Mo~6~I^i^~6~I^i^~2−x~Se~x~I^a^~6~ is based on a mixture of (x−1) [Mo~6~I^i^~6~Se^i^~2~I^a^~6~]^3−^ with (2−x) [Mo~6~I^i^~7~Se^i^I^a^~6~]^3−^. This leads to an average [Mo~6~I^i^~6~I^i^~2−x~Se~x~I^a^~6~]^3−^ ionic unit deduced from single‐crystal X‐ray diffraction investigations. The two inner positions of the average face‐capped [Mo~6~I^i^~8−x~Se^i^~x~I^a^~6~]^3−^ ionic units (located on the threefold axis of the unit) are randomly occupied by iodine and selenium, whereas the other ligand positions are fully occupied by iodine. Low‐temperature electron paramagnetic resonance (EPR) studies reveal a signal split into two components with g~∥~>g~⊥~. The reciprocal double integration intensity of the EPR signal versus T graph reveals a typical Curie law behavior. A density functional theory (DFT) study indicates that occupation of the inner position on the threefold axis by selenium atoms is preferred energetically among the three possible distributions of selenium atoms. The comparison of experimental and theoretical g values confirms the crystallographic analysis and agrees with the axial elongation of the Mo~6~ cluster within the crystal structure.