The dicationic W(TV) eta(2)-acetyl complex CpW(NCMe)(3)(eta(2)-COMe)(2+) (1) serves as the precursor for new tungsten(IV) eta(2)-acetyl derivatives, Treatment of 1 with halides provided trans-CpW(NCMe)(2)(X)(eta(2)-COMe)(+) (X = Cl, Br, I) in high yields. The reaction between CpW(NCMe)(2)(Br)(eta(2)-COMe)(+) and dppe yielded the 5-ring chelate CpW(eta(2)-dppe)(Br)(eta(2)-COMe)(+), while reactions with monodentate tertiary phosphines led to trans-CpW(PR(3))(2)(Br) (eta(2)-COMe)(+) via the intermediate CpW(NCMe)(PR(3))(Br) (eta(2)-COMe)(+) (PR(3) = PMePh(2), PPh(3)). Addition of acetonitrile to a nitromethane solution of CpW(PPh(3))(2)(Br)(eta(2)-COMe)(+) set up an equilibrium between the substrate and CpW(NCMe)(PPh(3))(Br)(eta(2)-COMe)(+). When AgBF4 was added, this ligand substitution reaction was accelerated. Reversible attachment of Ag+ at the 1 eta(2)-acetyl oxygen, ring opening, and reversible BF4- binding at the resulting vacant coordination site at W in trans-CpW(PMePh(2))(2)(Br)(eta(2)-COMe)(+) is suggested on the basis of H-1 and F-19 NMR spectroscopic observations. Similar steps are thought to be involved in the Ag+-catalyzed ligand substitution reaction. A single-crystal X-ray structure determination of CpW(NCMe)(2)(Br)(eta(2)-COMe)(+) revealed a four-legged piano stool structure in which the Br and eta(2)-acetyl ligands are located in a trans relationship and the C-O bond vector points away from the Cp ligand. Crystal data for 4(BF4-) : space group P (1) over bar, a = 7.782(2) Angstrom, b = 12.277(3) Angstrom, c = 16.173(4) Angstrom, alpha = 89.48(2)degrees, beta = 88.81(2)degrees, and gamma = 88.74(2)degrees. Least-squares refinement based on 4849 reflections converged to R = 0.056 and R(W) = 0.054.