Fluorocarbon sandwich compounds: Bis(octafluorocyclooctatetraene) derivatives of the first row transition metals

DFT studies on the early transition metal (C8F8)2M derivatives (M = Ti, V, Cr) predict the two C8F8 rings to have only 12 of the 16 carbon atoms of the two C8F8 rings bonded to the metal, thereby leading to 16-, 17-, and 18-electron metal configurations, respectively. The lowest energy structure of the titanium derivative Ti(C8F8)2 is predicted to have a singlet spin state with an octahapto and a tetrahapto C8F8 ring similar to the experimentally known hydrocarbon analog (η4-C8H8)Ti(η8-C8H8). However, the lowest energy structures for the vanadium and chromium analogs are bis(hexahapto) structures M(η6-C8F8)2 (M = V, Cr) rather than the octahapto–tetrahapto structures (η4-C8H8)M(η8-C8H8) predicted for the hydrocarbon analogs and found experimentally for V(C8H8)2. The transition metals from vanadium to iron are predicted to form tetrahapto–hexahapto derivatives (η4-C8F8)M(η6-C8F8) (M = V, Cr, Mn, Fe) in which 10 of the 16 carbon atoms of the two C8F8 rings are within bonding distance of the central metal. For the iron complex two types of tetrahapto bonding are found for the tetrahapto C8F8 ring in singlet (η4-C8F8)Fe(η6-C8F8) structures of essentially equal energies. For the late transition metal cobalt the lowest energy (C8F8)2Co structure is a doublet bis(tetrahapto) structure in which four adjacent carbon atoms of each C8F8 ring are bonded to the cobalt atom as a cis-diene. The lowest energy (C8F8)2Ni structure has only six carbon atoms of the two C8F8 rings within bonding distance of the nickel atom as two isolated CC double bonds in one ring and only a single CC double bond in the other ring. This gives the nickel atom only a 16-electron configuration similar to other zerovalent nickel complexes. The ligand exchange reactions 2C8H8 + (C8F8)2M = 2C8F8 + (C8H8)2M are predicted to be exothermic for all of the first row transition metals. This indicates that the C8F8 ligand is more weakly bonded to these metals than the C8H8 ligand in accord with the electron withdrawing effect of the ligand fluorine atoms.