The crystal and molecular structure of the title trinuclear complex has been determined from three-dimensional diffractometer data by Patterson and Fourier methods. Crystals are triclinic, space group Pi, a = 12.042(4), b = 20.359(11), c= 12.694(6) Â, α =101.51(4), 0 = 118.51(2), y = 92.20(4)° and isomorphous with the analogous copper complex. Data were refined by the least-squares procedure to R = 0.056 for 3732 independent reflections. There are two independent complex molecules in the unit cell with mercury atoms in two different centres of symmetry. The structure consists of linear Co...Hg...Co systems bridged by diethylphosphonate groups. Co...Hg separations amount to 4.078 Â in one and 4.065 Â in the other molecule. The Co atoms are surrounded by cyclopentadienyl rings and three Ρ atoms from diethylphosphonate groups. The mean values of the Co-C distances are 2.09 and 2.07 À, those of the Co -Ρ bond lengths 2.174 and 2.185 Â. The Hg atoms surrounded by six phosphonate-oxygens have fairly regular octahedral coordination with small (4+2) distortion. The mean values of the Hg-O bond lengths are 2.292 and 2.270 Â. The Ρ atom positions are disordered.