SYNTHESIS AND PROPERTIES OF THE 17-ELECTRON, TANTALUM-CENTERED RADICAL TA(CO)(4)(PH(2)PCH(2)CH(2)PPH(2))

The novel 17-electron compound Ta(CO)(4)dppe (dppe = Ph(2)PCH(2)CH(2)P Ph(2)) is formed via hydride hydrogen atom abstraction from TaH(CO)(4) dppe by the tris(p-tert-butylphenyl)methyl radical. The compound exist s in solution as an equilibrium mixture of monomer (the dominant speci es) and carbonyl-bridged dimer [Ta(CO)(4)dppe](2) but solely as the la tter in the solid state. It is very labile but was characterized elect rochemically, IR, Raman, and H-1 and P-31{H-1} NMR spectroscopically, and chemically. Typical of metal-centered radicals, Ta(CO)(4)dppe abst racts halogen atoms from organic halides RX to give the halotantalum c ompounds TaX(CO)(4)dppe and, in some cases, TaR(CO)(4)dppe. Cyclic vol tammetry experiments show that the oxidation of [Ta(CO)(4)(dppe)](-) i n CH2Cl2/0.1 M [Bu(4)N][PF6] is a one-electron process, irreversible a t 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 r adical is ascribed to reactions with the ionic medium, the main electr ochemical oxidation products being TaH(CO)(4)(dppe) and TaCl(CO)(2)(dp pe)(2).