Highly unsaturated binuclear butadiene iron carbonyls: quintet spin states, perpendicular structures, agostic hydrogen atoms, and iron-iron multiple bonds.

The highly unsaturated binuclear butadiene iron carbonyls (C(4)H(6))(2)Fe(2)(CO)(n) (n = 2, 1) have been examined using density functional theory. For (C(4)H(6))(2)Fe(2)(CO)(n) (n = 2, 1), both coaxial and perpendicular structures are found. The global minima of (C(4)H(6))(2)Fe(2)(CO)(n) (n = 2, 1) are the perpendicular structures 2Q-1 and 1Q-1, respectively, with 17- and 15-electron configurations for the iron atoms leading to quintet spin states. The Fe=Fe distance of 2.361 Å (M06-L) in the (C(4)H(6))(2)Fe(2)(CO)(2) structure 2Q-1 suggests a formal double bond. The Fe≡Fe bond distance in the (C(4)H(6))(2)Fe(2)(CO) structure 1Q-1 is even shorter at 2.273 Å (M06-L), suggesting a triple bond. Higher energy (C(4)H(6))(2)Fe(2)(CO)(n) (n = 2, 1) structures include structures in which a bridging butadiene ligand is bonded to one of the iron atoms as a tetrahapto ligand and to the other iron atom through two agostic hydrogen atoms from the end CH(2) groups. Singlet (C(4)H(6))(2)Fe(2)(CO) structures with formal Fe-Fe quadruple bonds of lengths ∼2.05 Å were also found but at very high energies (∼47 kcal/mol) relative to the global minimum.