Treatment of (TTP)Mo(PhC=CPh) (TTP = meso-tetra-p-tolylporphyrinato) with S8 or Cp2TiS5 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, P1, a = 12.735(2) Å,b = 13.521(2) Å, c = 13.920(2) Å, a = 74.76(1)·, β = 78.95(1)·, γ = 80.85(1)·, V = 2254.7(5) Å3, Z = 2, R = 3.1%, Rw = 4.2%). Complete sulfur atom transfer occurs between (TTP)Mo(PhC=CPh) and (TTP)Sn=S to give (TTP)Mo=S, (TTP)SnII, 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)SnII 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)SnII, and PhC=CPh. When treated with excess PPh3, the sulfido or selenido complex is reduced to (TTP)Mo(PPh3)2 with formation of either Ph3P=S or Ph3P=Se. However, when (TTP)Mo=S or (TTP)Mo=Se is reduced with PPh3 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 PPh3 < PhC=CPh < 4-picoline.