Abstract: According to the analysis of experimantal data, we found that:(l)crystal structure indicates that the title trans-molecule has a plane structure; (2)The vector sum of bond-moment of individual single, double and triple bond can not be used to explain the molecular dipole moment; (3)Raman spectrum misses characteristic frequency of vibration of double or triple bond; (4)Ultra-violet absorption λ_maxshifts toward red; (5) C-Cl bond in chloroethylene molecule and C-Hg bonds in diphenyl mercury molecule are proved to be conjugated. Thus we infer there exists a conjugated bond Ⅱ₅⁶in the title molecule including the vacant 6p_zorbital of mercury. In order to further study the formation of this conjugated bond,we applied MNDO semi-empirical molecular orbital calculation to these cis- and transmolecules.The results show that in each molecule, there exist some π molecular orbitals involving empty 6p_zorbital of mercury atom, the C-Hg bond has about 0.1 π-bond order,thus it proves the vacant 6p_zorbital of mercury indeed participates in the formation of conjugated system.By comparison of the difference of properties of cis- with trans- compound, we found that the cis- one has a higher melting poin,more negative heat of formation and the heat of elimination of C₂H₂from this molecule, less quantity in isotope exchange,its decompostion in alcohol is more slow, and its 1,4-reaction is more difficult to take place.In view of above distiction it shows that cis-molecule is more stable than trans-one. Thus we assumed that in cis-molecule there is a weak secondary chemical bond which located in the plane of molecule,essentially it is a weak three centers with four electrons delocalization bond. MNDO calculation supports this view point.In, in l964, the author already predicted the existence of this bond in abovemolecul.

Key words: Chlorovinyl mercury chloride, Conjugated bond, Secondary bond, MNDO