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 P2₁/n with Z = 4, a = 7.346(2) Å, b = 14.357(2) Å, c = 15.927(3) Å, and β = 101.17(2)°, while red-brown crystals of 6 form in space group C2/c with Z = 4, a = 14.416(3) Å,b = 12.829(2) Å,c = 8.982(2) Å, and β = 103.94(2)°. Full structures have been determined and show a close (3.58 Å) inter-ring Sb…Sb contact for 7, but no close As…As contact is found for 6. Extended Hückel MO calculations have been performed on the family of diheteroferrocenes (C₄H₄E)₂Fe (E = P, As, Sb, Bi).