Structures of metallocarboranes. V. Synthesis and crystal and molecular structure of the closo 20-electron bimetallocarborane 1,6-bis(.eta.-cyclopentadienyl)-1,6-diferra-2,3-dicarba-closo-decaborane(8), 1,6-(.eta.-C5H5)2-1,6,2,3-Fe2C2B6H8