Reaction of [MX(CO) 2 (η 7 -C 7 H 7 )] (M=Mo, X=Br; M=W, X=I) with two equivalents of CNBu t in toluene affords the trihapto-bonded cycloheptatrienyl complexes [MX(CO) 2 (CNBu t ) 2 (η 3 -C 7 H 7 )] (1, M=Mo, X=Br; 2, M=W, X=I). The X-ray crystal structure of 2 reveals a pseudo-octahedral molecular geometry with an asymmetric ligand arrangement at tungsten in which one CNBu t is located trans to the η 3 -C 7 H 7 ring. Treatment of 2 with tetracyanoethene results in 1,4-cycloaddition at the η 3 -C 7 H 7 ring to give [WI(CO) 2 (CNBu t ) 2 {η 3 -C 9 H 7 (CN) 4 }], 3. The principal reaction type of the molybdenum complex 1 is loss of carbonyl and bromide ligands to afford substituted products [MoBr(CNBu t ) 2 (η 7 -C 7 H 7 )] 4 or [Mo(CO)(CNBu t ) 2 (η 7 -C 7 H 7 )]Br. Reaction of [MoBr(CO) 2 (η 7 -C 7 H 7 )] with one equivalent of CNBu t in toluene at 60°C affords [MoBr(CO)(CNBu t )(η 7 -C 7 H 7 )], 5, which is a precursor to [Mo(CO)(CNBu t )(NCMe)(η 7 -C 7 H 7 )][BF 4 ], 6, by reaction with Ag[BF 4 ] in acetonitrile. In contrast with the parent dicarbonyl systems [MoX(CO) 2 (η 7 -C 7 H 7 )], complexes of the Mo(CO)(CNBu t )(η 7 -C 7 H 7 ) auxiliary, 5 and 6, do not afford observable η 3 -C 7 H 7 products by ligand addition at the molybdenum centre.