Phosphine-substituted (η⁵-pentadienyl) manganese carbonyl complexes: Geometric structures, electronic structures, and energetic properties of the associative substitution mechanism, including isolation of the slipped η³-pentadienyl associative intermediate

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.