Binuclear cyclopentadienylcobalt sulfur and phosphinidene complexes Cp 2Co 2E 2 (E = S, PX): Comparison with their Iron carbonyl analogues

Abstract

Density functional theory studies on a series of Cp 2Co 2E 2 derivatives (E = S and PX; X = H, Cl, OH, OMe, NH 2, NMe 2) 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 Cp 2Co 2S 2 and Cp 2Co 2(PH) 2 and are not found in the other systems. This differs from the corresponding Fe 2(CO) 6S 2 and Fe 2(CO) 6(PX) 2 derivatives where tetrahedrane structures are predicted to be the lowest energy structures for all cases except X = NR 2 and OH and such a tetrahedrane structure is found experimentally for Fe 2(CO) 6S 2. The butterfly structures for the Cp 2Co 2E 2 derivatives are of two types. For Cp 2Co 2(PX) 2 (X = H, OH, OMe, NH 2, NMe 2) the lowest energy structures are unsymmetrical butterflies Cp 2Co 2(P)(PX 2) with two X groups on one phosphorus atom and a lone pair on the other (naked) phosphorus atom. Related low-energy unsymmetrical butterfly Fe 2(CO) 6(P)(PX 2) structures, not observed in previous theoretical studies, are now found for the corresponding Fe 2(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 Cp 2Co 2(PX) 2 derivatives and are the global minima for Cp 2Co 2(PCl) 2 as well as Cp 2Co 2S 2. 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 Cp 2Co 2(PX) 2 derivatives but always at higher energies than the butterfly structures, ranging from 17 to 29 kcal/mol above the global minima.