The fluorophosphinidene ligand in cyclopentadienyliron carbonyl chemistry: Iron–iron multiple bonding versus higher spin states

Fluorophosphinidene (PF) is a versatile ligand found experimentally in the transient species M(CO)5(PF) (M = Cr, Mo) as well as the stable cluster Ru5(CO)15(μ4-PF). Theoretical studies on the cyclopentadienyliron carbonyl derivatives Cp2Fe2(PF)(CO)n (n = 4, 3, 2, 1, 0) predict the PF groups in the low-energy structures always to be bridging rather than terminal groups and almost always to be two-electron rather than four-electron donors. Viable structures are found for Cp2Fe2(PF)(CO)4 in which a PF group bridges two CpFe(CO)2 moieties without an Fe–Fe bond. Such structures are potentially accessible from NaFe(CO)2Cp and PF3. The low energy Cp2Fe2(PF)(CO)3 structures have one PF bridge and one CO bridge and are thus analogous to the well-known Cp2Fe2(CO)2(μ-CO)2. The lowest energy Cp2Fe2(PF)(CO)2 structure is a triplet spin state triply bridged structure analogous to the stable Cp2Fe2(μ-CO)3. Such structures have the two unpaired electrons of the triplet spin state in two orthogonal π “half-bond” components in the formal FeFe double bond. Higher spin states, even quintets and septets, are energetically preferred over Fe–Fe triple and quadruple bonds for the highly unsaturated Cp2Fe2(PF)(CO)n (n = 1, 0). In addition Cp2Fe2(PF)(CO)n (n = 1, 0) structures are found in which the P–F bond of the PF ligand has split to form separate P and F ligands.