Treatment of (TTP)Mo(PhC=CPh) (TTP = meso-tetra-p-tolylporphyrinato) with S-8 or Cp(2)TiS(5) in toluene results in the formation of(TTP)Mo=S. The sulfide complex may also be prepared from the treatment of (TTP)MoCl2 with Li2S in THF.. Similarly, a terminal selenide complex may be prepared from treatment of (TTP)MoCl2 with Na2Se. The structure of (TTP)Mo=S has been determined by single-crystal X-ray diffraction analysis (triclinic, P ($) over bar 1, a = 12.735(2) Angstrom, b = 13.521(2) Angstrom, c = 13.920(2) Angstrom, alpha = 74.76(1)degrees, beta = 78.95(1)degrees, gamma = 80.85(1)degrees, V = 2254.7(5) Angstrom(3), Z = 2, R = 3.1%, R(W) = 4.2%). Complete sulfur atom transfer occurs between (TTP)Mo(PhC=CPh) and (TTP)Sn=S to give (TTP)Mo=S, (TTP)Sn-II, and PhC=CPh. The net result is a formal two-electron redox process that occurs irreversibly between Sn(IV) and Mo(II). Correspondingly, no reaction is observed between (TTP)Mo=S and (TTP)Sn-II in the presence of excess PhC=CPh. The analogous treatment of (TTP)Mo(PhC=CPh) with (TTP)Sn=Se results in the reversible exchange of a selenium ligand to form (TTP)Mo=Se, (TTP)Sn-II, and PhC=CPh. When treated with excess PPh(3), the sulfide or selenido complex is reduced to (TTP)Mo(PPh(3))(2) with formation of either Ph(3)P=S or Ph(3)P=Se. However, when (TTP)Mo=S or (TTP)Mo=Se is reduced with PPh(3) in the presence of PhC=CPh, (TTP)Mo(PhC=CPh) is formed. The alkyne ligand of (TTP)Mo(PhC=CPh) may be displaced by 4-picoline to give trans-(TTP)Mo(4-picoline)(2). Ligand preference for the porphyrin Mo(II) center is thus PPh(3) < PhC=CPh < 4-picoline.