The strain energies and the pyramidalization parameters of bicyclo [3-.3.1]nonylydenebicyclo[3.3.1]nonane(BBN), trans-(1-methyl-2-bicyclo[3.3.4]-1-methyl bicyclo [3.3.1]nonane (DMBBN) and trans-(1-tert-butyl-2-bicyclo[3.3.1])-1-tert-butylbicyclo [3.3.1] nonane (DBBBN) molecules have been-calculated using MM2 and AMBER molecular mechanic methods. The strain energy and the pyramidalization degree of the molecule are increased by increasing the volume of R groups at the allylic position of the double bond. The electronic structure of the molecules have been investigated by semiempirical PM3 method. The results of molecular mechanic and PM3 methods, agree with the X-ray results and shows that the double bond of BBN molecule has a planar structure. The 1:1 pi-molecular complexes of the molecules formed with Br-2 ate investigated using PM3 method and it is seen that their stable configuration ha's an axial structure. The electronic and the steric factors affecting the structure and the stability of the molecular complexes have been studied. It is found that the DMBBN...Br-2 complex is more stable than the BBN...Br-2 complex. It is determined that DBBBN...Br-2 complex containing the bulky tert-butyl group has the lowest stabilization energy among the investigated molecular complexes.