Decarbonylation Products of Binuclear Methylphosphinidene Complexes of Cyclopentadienyliron Carbonyls: Triplet and Quintet Structures Are Favored Energetically over Singlet Structures with Iron–Iron Multiple Bonding

The structures, energetics, and energetically preferred
spin states
of methylphosphinidene-bridged binuclear cyclopentadienyliron carbonyl
complexes MePFe2(CO)nCp2 (n = 4, 3, 2, and 1) related to the experimentally
known (μ-RP)Fe2(μ-CO)(CO)2Cp2 (R = cyclohexyl, phenyl, mesityl, and 2,4,6-tBu3C6H2) complexes have been investigated by density
functional theory. Singlet structures having a pyramidal pseudotetrahedral
phosphorus environment with 18-electron iron configurations are energetically
preferred in the tricarbonyl and tetracarbonyl systems MePFe2(CO)nCp2 (n = 4 and 3) with the lowest energy structures of the tricarbonyl
very closely resembling the experimentally determined structures.
For the more unsaturated dicarbonyl and monocarbonyl systems MePFe2(CO)nCp2 (n = 2 and 1), higher spin state triplet and quintet structures are
energetically preferred over singlet structures. These more highly
unsaturated structures can be derived from the lowest energy singlet
MePFe2(CO)nCp2 (n = 4, 3) by the removal of carbonyl groups. The iron atoms
giving up carbonyl groups in their 16- and 14-electron configurations
bear the spin density of the unpaired electrons in the higher spin
states. The lowest energy singlet structure of the monocarbonyl MePFe2(CO)Cp2, although a relatively high energy isomer,
is unusual among the collection of MePFe2(CO)nCp2 (n = 4, 3, 2, and
1) structures by having both the formal FeFe double bond and
the four-electron donor MeP unit with the planar phosphorus coordination
required to allow each of its iron atoms to attain the favored 18-electron
configuration.