The solvothermal reaction of cobalt(II) acetate with p-tert-butylcalix[ 8] arene ( calix) and triethylamine affords the compound (Et3NH)(2) [Co-2(II)(calix)(2)] (1 center dot 2Et(3)NH) that shows a hydrogen bond bridged dinuclear complex [Co-2(II)(calix)(2)](2-) ( 1) with cobalt( II) ions in a tetrahedral geometry. The compound crystallises in the monoclinic, space group P2(1)/n with cell parameters a = 14.89( 1) angstrom, b = 20.90( 2) angstrom, c = 30.87( 4) A, beta = 102.57( 7)degrees. V = 9376( 16) angstrom(3), Z = 2. The magnetic studies together with ab initio calculations are evidence of an important role of the geometry of the second coordination sphere of carbon and hydrogen atoms around the CoO4 core in quantifying the zero field splitting on cobalt sites. This results in strong magnetic anisotropies with a negative axial component on the cobalt fragments.