Reaction of 2, 2′: 6′, 2″-terpyridyl (tpy) with ruthenium dicarbonyl dihalides yields the complexes Ru(CO)2X2(tpy) (X = Br or Cl), which can be protonated giving [RU(CO)2X2(tpyH)] ClO4. Crystal structures of the two forms (red and yellow) of Ru(Co)2Br2(tpy) show each to have octahedral stereochemistry with cis-carbonyls, trans-bromines, and bidentate tpy. Treatment of Ru(CO)2X2(tpy) complexes with trimethylamine N-oxide in dichloromethane at room temperature gives cis-Ru(CO)X2(tpy) complexes. The presence of cis halogens and tridentate terpyridyl in the chloro complex has been established by X-ray crystallography. Reaction of terpyridyl with ruthenium trichloride in dimethylformamide yields trans-Ru(CO)Cl2(tpy), the crystal structure of which has been determined.