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{ηx-(Me3Si) 2C3H3}3]- anion and its parent anion [Mn(ηx-C3H5)3] - in the high-spin or S = 5/2 state at the BP86/AE1 level of theory yielded the mixed hapticity structures [Mn{η3-(Me 3Si)2C3H3}{η 1-(Me3Si)2C3H3) 2}]- and [Mn(η3-C3H 5)(η1-C3H5)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 η3-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 η3-bonded ligands, while there is increased covalent character in the bonds to the η1-bonded ligands. Investigations of the related intermediate-spin forms of these complexes for which S = 3/2 afford structures of composition [Mn{η3-(Me3Si)2C 3H3}2{η1-(Me3Si) 2C3H3) }]- (1-IS) and [Mn(η 3-C3H5)2(η1-C 3H5)]- (2-IS), in which one of the allyl ligands has slipped from an η1- to η3-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-(Me3Si)2C3H 3}{η1-(Me3Si)2C 3H3)2}], [Li-(thf)4][1], yielded a Curie constant that indicated the presence of an 5 = 3/2 spin state for the Mn(II) ion. © 2006 American Chemical Society.