The reaction of mercuric acetate with 2,9-diphenyl-1,10-phenanthroline (dpp) in refluxing ethanol and in the presence of lithium chloride gave the cyclometalated complex [Hg(C AND N-dpp)Cl]2. X-ray crystal analysis revealed a dimeric molecular structure : [HgC24H15N2Cl]2, fw = 567.48 X 2, P2(1)/n, a = 11.79(1) angstrom, b = 17.80(1) angstrom, c = 18.84(1) angstrom, beta = 99.23(6)-degrees, V = 3904(5) angstrom3, Z = 4. The long Hg-N distances (2.60-3.85 angstrom) and proton chemical shifts of the metalated dpp suggest that the 1,10-phenanthroline ring is weakly coordinated to the Hg-phenyl moiety. The complex is luminescent at both 77 and 298 K. Photosensitivity of [Hg(C A N-dpp)Cl] 2 at room temperature is evidenced by its facile transformation into the protonated salt of dpp (Hdpp+) in dichloromethane. The low-temperature emission spectrum of [Hg(C AND N-dpp)Cl] 2 is almost identical to that of dpp. The photophysical and emission properties of [Hg(C AND N-dpp)Cl]2 are compared with those of the related cyclometalated Pt(II) and Au-(III) complexes, viz. [Pt(C AND N AND N-dpp)(NCCH3)]+ and [Au(C AND N AND N-dpp)Cl]+.