The syntheses and properties of tetra- and pentanuclear vanadium(IV, V) carboxylate complexes are reported. Reaction of (NBzEt(3))(2)[VOCl4] (1a) with NaO(2)CPh and atmospheric H2O/O-2 in MeCN leads to formation of (NBzEt(3))(2)[V5O9Cl(O(2)CPh)(4)] 4a; a similar reaction employing (NEt(4))(2)[VOCl4] (1b) gives (NEt(4))(2)[V5O9Cl(O-2-CPh)(4)] (4b). Complex 4a . MeCN crystallizes in space group P2(1)2(1)2(1) with the following unit cell dimensions at -148 degrees C : a = 13.863(13) Angstrom, b = 34.009(43) Angstrom, c = 12.773(11) Angstrom, and Z = 4. The reaction between (NEt(4))(2)[VOBr4] (2a) and NaO(2)CPh under similar conditions gives (NEt(4))(2)[V5O9Br(O(2)CPh)(4)] (6a), and the use of (PPh(4))(2)[VOBr4] (2b) likewise gives (PPh(4))(2)[V5O9Br(O(2)CPh)(4)] (6b). Complex 6b crystallizes in space group P2(1)2(1)2(1) with the following unit cell dimensions at -139 degrees C : a = 18.638(3) Angstrom, b = 23.557(4) Angstrom, c = 12.731(2) Angstrom, and Z = 4. The anions of 4a and 6b consist of a V-5 square pyramid with each vertical face bridged by a mu(3)-O2- ion, the basal face bridged by a mu(4)-X(-) (X = Cl, Br) ion, and a terminal, multiply-bonded O2- ion on each metal. The RCO(2)(-) groups bridge each basal edge to give C-4v virtual symmetry.