The novel 17-electron compound Ta(CO)4dppe (dppe = Ph2PCH2CH2PPh2) is formed via hydride hydrogen atom abstraction from TaH(CO)4dppe by the tris(p-tert-butylphenyl)methyl radical. The compound exists in solution as an equilibrium mixture of monomer (the dominant species) and carbonyl-bridged dimer [Ta(CO)4dppe]2 but solely as the latter in the solid state. It is very labile but was characterized electrochemically, IR, Raman, and 1H and 31P{1H} NMR spectroscopically, and chemically. Typical of metal-centered radicals, Ta(CO)4dppe abstracts halogen atoms from organic halides RX to give the halotantalum compounds TaX(CO)4dppe and, in some cases, TaR(CO)4dppe. Cyclic voltammetry experiments show that the oxidation of [Ta(CO)4(dppe)]- in CH2Cl2/0.1 M [Bu4N][PF6] is a one-electron process, irreversible at scan rates below 50 V/s. The formal potential of [Ta(CO)4(dppe)]0/- is estimated as −1.2 V vs ferrocene. The reduced lifetime of the radical is ascribed to reactions with the ionic medium, the main electrochemical oxidation products being TaH(CO)4(dppe) and TaCl(CO)2(dppe)2.

Reductive carbonylation of TaCl(5) in the presence of 1,2-bis(diphenylphosphino)ethane (dppe) under the appropriate conditions results in the formation of TaCl(CO)(2)(dppe)(2) (1), as the major product, and the possibly cyclic oligomer [TaCl(CO)(2)(dppe)(2)](2)(x)() (2, 2x >/= 4) as a minor product. Carbonylation of 1 (1 atm) results in the rapid but reversible formation of TaCl(CO)(4)(dppe) (3). Solutions of all three compounds exhibit low levels of paramagnetism, possibly attributable to thermal population of low-lying triplet excited states. Crystal data for the toluene solvate of 1, C(68)H(64)ClO(2)P(4)Ta: triclinic, P&onemacr; (No. 2), a = 13.937(12) Å, b = 14.811(7) Å, c = 14.929(9) Å, alpha = 102.30(5) degrees, beta = 95.60(7) degrees, gamma = 98.41(5) degrees, Z = 2.