A compound containing two tantalum atoms in oxidation states separated by six units

Reaction of [Et4N][Ta(CO)6] (1) and Hhpp, Hhpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, gives a mixed-valent tantalum compound [Ta(hpp)4][Ta(CO)6] (2). It contains the eight-coordinate Ta(V) cation [Ta(hpp)4]+, accompanied by the homoleptic hexacarbonyltantalate(−I) anion from the starting material. Thus, the oxidation numbers for the tantalum atoms span a range of six units. This is the first complex containing four chelating hpp anions. Crystal structures of 1, 2 and [Ph4P][Ta(CO)6] (3) were determined by X-ray crystallography.