The pentahapto-bonded cyclooctadienylmolybdenum complexes [Mo(CO)₂(L₂)(1–3:5,6-η-C₈H₁₁)][BF₄] (L₂ = Ph₂P(CH₂)_nPPh₂, n = 1, (dppm), 1a; n = 2, (dppe), 2a) are synthesised by reaction of [MoBr(CO)₂(L₂)(1–3-η:5,6-C₈H₁₁)] with Ag[BF₄] in CH₂Cl₂; [Mo(CO)₂(dppm)(1–5-η-C₈H₁₁)][BF₄], 1b, is obtained from [MoBr(CO)₂(dppm)(1–3-η:4,5-C₈H₁₁)] by an identical procedure. Related syntheses afford [Mo(CO)₂{P(OMe)₃}₂(1–5-η-C₈H₁₁)][BF₄], 4b, and [Mo(CO)₂(CNBu^t)₂(1–3:5,6-η-C₈H₁₁)][BF₄], 5a. The complexes [Mo(CO)₂(NCMe)₃(η³-C₈H₁₁)]⁺ (η³-C₈H₁₁ = 1–3-η:5,6-C₈H₁₁, 1–3-η:4,5-C₈H₁₁) are precursors to [Mo(CO)(CNBu^t)₃(1–3:5,6-η-C₈H₁₁)][BF₄], 6a, [Mo(CO)(CNBu^t)₃(1–5-η-C₈H₁₁)][BF₄], 6b and [Mo(CO)₂(norborna-2,5-diene)(1–3:5,6-η-C₈H₁₁)][BF₄], 7a. The X-ray crystal structures of complexes 1a and 6b have been determined. NMR spectroscopic investigations on the 1–5-η-C₈H₁₁ complexes 1b, 4b and 6b, indicate a high barrier to rotation of the metal group with respect to the cyclooctadienyl ring. Complexes 1a, 1b and 2a undergo ligand addition reactions with accompanying η⁵ → η³ hapticity conversion at the cyclooctadienyl ligand to yield adducts [Mo(CO)₂(L′)(L₂)(η³-C₈H₁₁)]⁺ (L′ = NCMe or CO, L₂ = dppm or dppe, η³-C₈H₁₁ = 1–3-η:5,6-C₈H₁₁ or 1–3-η:4,5-C₈H₁₁, not all combinations). The facility of these processes is strongly dependent upon the identity of the variables L₂ and C₈H₁₁.