Binuclear cyclopentadienylcobalt sulfur and phosphinidene complexes Cp2Co2E2 (E = S, PX): Comparison with their Iron carbonyl analogues

Density functional theory studies on a series of Cp2Co2E2 derivatives (E = S and PX; X = H, Cl, OH, OMe, NH2, NMe2) predict global minimum butterfly structures with one Co–Co bond for the “body” of the butterfly and four Co–E bonds at the edges of the “wings” of the butterfly. Tetrahedrane structures with both Co–Co and E–E bonds are higher in energy for Cp2Co2S2 and Cp2Co2(PH)2 and are not found in the other systems. This differs from the corresponding Fe2(CO)6S2 and Fe2(CO)6(PX)2 derivatives where tetrahedrane structures are predicted to be the lowest energy structures for all cases except X = NR2 and OH and such a tetrahedrane structure is found experimentally for Fe2(CO)6S2. The butterfly structures for the Cp2Co2E2 derivatives are of two types. For Cp2Co2(PX)2 (X = H, OH, OMe, NH2, NMe2) the lowest energy structures are unsymmetrical butterflies Cp2Co2(P)(PX2) with two X groups on one phosphorus atom and a lone pair on the other (naked) phosphorus atom. Related low-energy unsymmetrical butterfly Fe2(CO)6(P)(PX2) structures, not observed in previous theoretical studies, are now found for the corresponding Fe2(CO)6(PX)2 derivatives. Symmetrical butterfly singlet diradical structures with one X group on each phosphorus atom in relative cis or trans positions are also found for the Cp2Co2(PX)2 derivatives and are the global minima for Cp2Co2(PCl)2 as well as Cp2Co2S2. In all cases the cis structures are of lower energy than the corresponding trans structures. Rhombus structures having neither Co–Co nor E–E bonds are also found for all of the Cp2Co2(PX)2 derivatives but always at higher energies than the butterfly structures, ranging from 17 to 29 kcal/mol above the global minima.