The addition of various nucleophiles (Nu1) to the cationic complex [CpRu(η6-cot)]+ (1+) has been performed and yielded the neutral complexes CpRu(η5-C8H8Nu1) (2) [cot = cyclooctatetraene; Nu1 = Me (2a), CH(CO2Me)2 (2b), CH=CH2 (2c), C6H5 (2d), H (2e), D (2f), NMe2 (2g), OMe, (2h), C≡CtBu (2i)]. The nucleophiles Nu1 add exclusively to the cot ring, and stereoselectively in the exo position with respect to the metal center. As evidenced by NMR spectroscopy the cyclo-C8 ligand in 2c, 2e−2i is bound in a 1,2-η:5,6,7-η fashion in solution. The molecular structure analysis of 2c [space group P1¯, a = 757.9(1), b = 759.1(1), c = 1053.1(3) pm, α = 81.93(2), β = 80.22(1), γ = 88.73(1)°, V = 591.2(2) A3, Z = 2, Rmerge = 0.0265] verifies the 1,2-η:5,6,7-η coordination mode of the cyclo-C8 ligand in the solid state and shows the Nu1 substituent to be in an exo position. For 2a and 2b a 1,2,3,4,5-η haptomer is also found, and this interconverts to the 1,2-η:5,6,7-η derivative in solution; in contrast, the cyclo-C8 ligand of the phenyl derivative 2d remains in the 1,2,3,4,5-η fashion. The protonation of the neutral complexes 2a−2d by the addition of HBF4 generates the new cationic products [CpRu(η6-C8H9Nu1)]+ (3+) [Nu1 = Me (3a+), CH(CO2Me)2 (3b+), CH=CH2 (3c+), C6H5 (3d+)]. When Nu1 = CH(CO2Me)2 (3b+) two different haptomers were isolated that exhibit 1,2-η:4,5,6,7-η and 1,2,3,4,5,6-η bonding modes, respectively. In solution the 1,2-η:4,5,6,7-η haptomer completely rearranges to the 1,2,3,4,5,6-η derivative. The crystal structure analysis of 3dBF4 [space group Pbc2(1), a = 1678.8(7), b = 1031.4(2), c = 981.8(2) pm, V = 1700.0(9) A3, Z = 4, Rmerge = 0.032] confirms the 1,2-η:4,5,6,7-η bonding mode of the cyclo-C8 ligand, which has also been indicated in solution. The second nucleophilic addition to the coordinated cyclo-C8 ligand yields new neutral complexes that exist as two different haptomers depending on the steric demand of the two nucleophiles. The haptomer CpRu(1,2,3,4,5-η-C8H9-6-Nu1-8-Nu2) (4a) is formed with Nu1 = Nu2 = CH(CO2Me)2 whereas with Nu1 = Me, Nu2 = CH(CO2Me)2 (4b) and Nu1 = Nu2 = Me (4c) the haptomer CpRu(1,2-η:5,6,7-η-C8H9-4-Nu1-8-Nu2) is obtained. The protonation of 4a−4c produces the cationic hydride species [CpRu(H)(1,2,3,4,5-η-C8H9Nu1Nu2)]+ (5+) [Nu1 = Nu2 = CH(CO2Me)2 (5a+), Nu1 = CH3, Nu2 = CH(CO2Me)2 (5b+), Nu1 = Nu2 = Me (5c+)] as indicated by the high-field shifted singlet for one proton below δ = −10 in the 1H-NMR spectra. An X-ray structure analysis of a single crystal obtained from the protonation reaction of 4b [space group P21/c, a = 1285.5(12), b = 1015.8(8), c = 1586.3(14) pm, β = 108.61(7)°, V = 1963(3) A3, Z = 4, Rmerge = 0.0942] reveals a special disorder, the interpretation of which results in the structure determination of two independent complexes in a ratio of 59:41 differing by two hydrogen atoms in the formula. Both complexes are cations; one exhibits a disubstituted 1,2,3,4,5-η-bonded cyclooctadienyl ligand and an agostic interaction of the Ru center with a hydrogen atom at C5 of the cyclo-C8 ligand (5bBF4), whereas the second complex contains a cyclooctatriene ligand in a 1,2,3,4,5,6-η bonding mode without a metal hydride function (6BF4). The coexistence of two different molecules in one single crystal that differ by two hydrogen atoms in addition to the agostic hydrogen atom in 5b+, points to an elimination of an H2 molecule from 5b+in the solid state to generate 6+. This suggestion is corroborated by the formation of a cationic complex 7+as the main product in solution, which contains a cyclo-C8 ligand without the malonate nucleophile [Nu2 = CH(CO2Me)2], whereas a freshly prepared NMR sample only reveals 5b+and 6+. In all of these products the Me nucleophile (Nu1 = Me) is linked to a metal-coordinated carbon atom indicating an additional intramolecular hydrogen migration after the initial protonation.