SECONDARY BONDING IN ORGANOPNICOGEN COMPOUNDS - COMPARISON OF THE STRUCTURES OF OCTAMETHYL-1,1'-DISTIBAFERROCENE AND OCTAMETHYL-1,1'-DIARSAFERROCENE

2,2',3,3',4,4',5,5'-Octamethyl-1,1'-diarsaferrocene (6) and 2,2',3,3',4,4',5,5'-octamethyl-1,1'-distibaferrocene (7) were obtained from the reactions of the corresponding 1-phenyl-2,3,4,5-tetramethylheteroles with lithium followed by FeCl2. Red-black crystals of 7 form in space group P2(1)/n with Z = 4, a = 7.346(2) angstrom, b = 14.357(2) angstrom, c = 15.927(3) angstrom, and beta = 101.17(2)-degrees, while red-brown crystals of 6 form in space group C2/c with Z = 4, a = 14.416(3) angstrom, b = 12.829(2) angstrom, c = 8.982(2) angstrom, and beta = 103.94(2)-degrees. Full structures have been determined and show a close (3.58 angstrom) inter-ring Sb...Sb contact for 7, but no close As...As contact is found for 6. Extended Huckel MO calculations have been performed on the family of diheteroferrocenes (C4H4E)2Fe (E = P, As, Sb, Bi).