Crystal and molecular structure of (η⁵-1,3,5-trimethyl-6- phenylcyclohexadienyl)(η⁵-cyclopentadienyl)iron, a mixed full sandwich iron(II) complex

(η⁵-1,3,5-trimethyl-6-phenylcyclohexadienyl)(η⁵-cyclopentadienyl)Fe (9) was synthesized via reaction of phenyllithium with [(cyclopentadienyl)(mesitylene)Fe][PF₆]. 9 crystallized from hexanes in the monoclinic space group P2₁/n with a 8.5703(5), b 8.5283(9), c 21.8436(23) Å, β 93.868(6)° and D_calc 1.327 g cm^-3 for Z = 4. Least-squares refinement gave a conventional R value of 0.035 for 2046 independent observed reflections. The structure reveals that the phenyl moiety added in the expected exo fashion and that the Fe-ring distances are statistically identical, averaging 2.059(3) and 2.06(3) Å for the cyclopentadienyl and cyclohexadienyl moieties, respectively. A comparison with other mixed iron(II) full-sandwich complexes is made in an attempt to determine relative π-back bonding abilities of the four isoelectronic π-ligands: arene, cyclopentadienyl, pentadienyl, and cyclohexadienyl.