The molecule (η5-Me2Pdl)Mn(CO)3 (η5-Me2Pdl = 2,4-dimethyl-η5-pentadienyl) has been
prepared by a new method and
used as a starting material to prepare
the molecules (η5-Me2Pdl)Mn(CO)
n
(PMe3)3–n
(n = 2, 1) by phosphine substitution for
carbonyls. The first carbonyl substitution is achieved thermally in
refluxing cyclohexane, and the second carbonyl substitution requires
photolysis. At room temperature in benzene the associative intermediate
(η3-Me2Pdl)Mn(CO)3(PMe3) that precedes the initial loss of carbonyl is observed.
Single-crystal structures are reported for all complexes, including
the associative intermediate of the first substitution in which the
pentadienyl ligand has slipped to the η3 bonding
mode. These molecules offer an opportunity to examine fundamental
principles of the interactions between metals and pentadienyl ligands
in comparison to the well-developed chemistry of metal cyclopentadienyl
(Cp) complexes as a function of electron richness at the metal center.
Photoelectron spectra of these molecules show that the Me2Pdl ligand has π ionizations at energy lower than that for
the analogous Cp ligand and donates more strongly to the metal than
the Cp ligand, making the metal more electron rich. Phosphine substitutions
for carbonyls further increase the electron richness at the metal
center. Density functional calculations provide further insight into
the electronic structures and bonding of the molecules, revealing
the energetics and role of the pentadienyl slip from η5 to η3 bonding in the early stages of the associative
substitution mechanism. Computational comparison with dissociative
ligand substitution mechanisms reveals the roles of dispersion interaction
energies and the entropic free energies in the ligand substitution
reactions. An alternative scheme for evaluating the computational
translational and rotational entropy of a dissociative mechanism in
solution is offered.
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