Structure, Bonding, and Paramagnetism in the Manganese(II) Tris-Allyl Anions [Mn{ηx-(C3H3R2)3}]- (R = H, SiMe3; x = 1 or 3): Insight from Theory

Geometry optimizations of the [Mn{η x -(Me 3 Si) 2 C 3 H 3 } 3 ] - anion and its parent anion [Mn(η x -C 3 H 5 ) 3 ] - in the high-spin or S = 5/2 state at the BP86/AE1 level of theory yielded the mixed hapticity structures [Mn{η 3 -(Me 3 Si) 2 C 3 H 3 }{η 1 -(Me 3 Si) 2 C 3 H 3 ) 2 }] - and [Mn(η 3 -C 3 H 5 )(η 1 -C 3 H 5 ) 2 ] - ( 1-HS and 2-HS, respectively). These calculations revealed that the silyl substituents in 1-HS have very little influence on the hapticities of the ligands and the asymmetric bonding of the η 3 -bonded ligand, suggesting that these structural features are intrinsic to the complex. Natural population analyses and calculations of Wiberg bond indices for 1-HS indicate predominantly ionic bonding between manganese and the η 3 -bonded ligands, while there is increased covalent character in the bonds to the η 1 -bonded ligands. Investigations of the related intermediate-spin forms of these complexes for which S = 3/2 afford structures of composition [Mn{η 3 -(Me 3 Si) 2 C 3 H 3 } 2 {η 1 -(Me 3 Si) 2 C 3 H 3 ) }] - ( 1-IS ) and [Mn(η 3 -C 3 H 5 ) 2 (η 1 -C 3 H 5 )] - ( 2-IS ), in which one of the allyl ligands has slipped from an η 1 - to η 3 -bonding mode with a concomitant decrease in the manganese−carbon distances, and which again reveals essentially no dependency of hapticity upon the spatial requirements of the silyl substituents. The relative energies of 1-HS, 1-IS, and the low-spin state 1-LS (for which S = 1/2) at both the BP86/AE1 and B3LYP/AE1 levels of theory revealed 1-HS and 1-LS to be the most and least stable forms of 1, respectively, with the energy of 1-LS being prohibitively high at each level of theory. A reinvestigation of the experimental magnetic susceptibility of the ion-separated species [Li(thf) 4 ][Mn{η 3 -(Me 3 Si) 2 C 3 H 3 }{η 1 -(Me 3 Si) 2 C 3 H 3 ) 2 }], [Li(thf) 4 ][ 1 ], yielded a Curie constant that indicated the presence of an S = 3/2 spin state for the Mn(II) ion.