The synthesis, crystal structure and magnetic properties of the title compound are presented : Fe[C-5(CH3)(5)](2)[Ni(III)(S4C3O)(2)](CH3CN), Mr = 786.65, crystallizes in the C2/m monoclinic space group with cell parameters a=16.374(4) Angstrom, b=10.84(2)Angstrom, c=19.530(5)Angstrom, beta=98.02(2)degrees, V = 3431 Angstrom(3), Z=4, d(calc)= 1.523 g . cm(-3), Full-matrix least-squares refinements gave R(F)=0.060, Rw(F)=0.077 based on 1068 reflections with I>3 sigma(I). The magnetic behavior is typical of dominant ferromagnetic interactions to which very weak antiferromagnetic interactions are superimposed.