Abstract: The single crystal of [Et₄N]₄ [MO₂Fe₄S₉(SCH₂CH₂S)₂] was obtainedby reaction of (NH₄)₂HoS₄, FeCl₃, HSCH₂CH₂SH and Et₄ NBr in CH₃CN and MeOH-MeONa. Cluster compound [Et₄N]₄[Mo₂Fe₄S₉(SCH₂CH₂S)₂] crystal -Lized in the monoclinic space group C₂/c with unit cell parameters:a = 17. 672(2)Å, b = 33. 851(4)Å, c = l3. 900(3)Å, β=135.11(4)°, V=5868.8 (3.9)ų and Z = 4. On the basis of 3514 unique data (I>2σ(I)) the structure was refined to R = 0.0575. The anion [Mo₂Fe₄S₉(SCH₂CH₂S)₂]^4- of the cluster compound includes doubly bridging (μ₂-S), triply bridging (μ₃-S) and quadruply bridging (μ₄ -S) . The structrue analysis gives a rule with bond lengths decreasing in the order of Fe-(μ₄-S)> Fe-(μ₃-s)> Fe-(μ₂ -s) and EHMO calculation gives the other rule with bond order increasing in the order of Fe-(μ₄-S)< Fe-(μ₃-S)<Fe-(μ₂-S). The experimental and theoretical results are in good agreement. According to the nearby energy level arrangement of front orbit the author considers that it has six unpaired electrons and spin of 3. The measurement of magnetic moment proved that the result of EHMO calculation is correct On the basis of summarization on structural parameters of relative Mo-Fe-S cluster compound and the data of Mo(Fe) valence state the authors suggest that the terminal bond of Mo-St is a more effective monitor of oxidation states and the Mo and Fe atoms in the title cluster compound have oxidation states of +5 and +2 respectively. The cluster anion [Mo₂Fe₄S₉(SCH₂CH₂S)₂]^4- isarranged in a structure of idealized C₂ symmetry in which the C₂ axis contains μ₄ -S (7) . There are three types in cation Et₄N⁺: (1) general position, (2) special position, C₂ axis passes through N atom,(3) special position, C₂ axis passes through a terminal C. The local disorder of Et₂N⁺ in special position appears as result of requirement of C₂ axis.

Key words: Mo-Fe-S cluster compound, single crystal, Monoclinic space group, anion, Structureal parameters, Valence state, Oxidation state, Unit cell parameter