Replacement of CO ligands in the η 3-allyl complex [Cp*Re(CO) 2(η −C 3H 5)][BF 4] 1 has been accomplished by reacting 1 in acetonitrile with PhIO or Me 3NO to yield endo isomers of [Cp*Re(CO)(NCMe)(η 3-C 3H 5)][BF 4] 2 and [Cp*Re(NCMe) 2(η 3-C 3H 5)][BF 4] 3 respectively. The assignment of the allyl proton resonances and the isomeric forms of 2 and 3 have been deduced with the aid of 1H nuclear magnetic resonance nuclear Overhauser enhancement experiments. The X-ray crystal structures of 2 and 3 confirm that these are endo isomers. Poor quality crystals of 2 (with 0.5 toluene solvate) are monoclinic, of space group P2 1/ c, with Z = 4, a = 8.381(2) Å, b = 15.389(2) Å c = 17.161(3) Å, β = 91.71(2)° and V= 2212.4 Å 3. The structure refined to R F = 0.060 for 1950 data ( I o ⩾ 2.5σ(I o)). Crystals of 3 are orthorhombic, of space group P2 12 12 1, with Z = 4, a = 11.296(2) Å, b = 12.012(2) Å, c = 14.760(2) Å and V= 2002.7 Å 3. The structure refined to R F = 0.027 for 1815 data ( I o >/ 2.5σ( I o)). The rhenium atom is bonded to the allyl group in 3 with an ReC(7) distance of 2.124(11) Å to the central carbon atom and an ReC(6) distance of 2.177(12) Å and an Re-C(8) distance of 2.201(12) Å to the terminal carbon atoms. The acetonitrile groups are bound with an ReN(1) distance of 2.089(7) Å and an ReN(2) distance of 2.086(10) Å, and NC with an N(1)C(9) distance of 1.125(11) Å and an N(2)C(11) distance of 1.134(14) Å.