| Abstract
| - 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-(Me3Si)2C3H3}{η1-(Me3Si)2C3H3)2}]- and [Mn(η3-C3H5)(η1-C3H5)2]- (1-HS and 2-HS, respectively). These calculations revealed that the silyl substituents in 1-HS have very little influence on thehapticities of the ligands and the asymmetric bonding of the η3-bonded ligand, suggesting that thesestructural features are intrinsic to the complex. Natural population analyses and calculations of Wibergbond indices for 1-HS indicate predominantly ionic bonding between manganese and the η3-bondedligands, while there is increased covalent character in the bonds to the η1-bonded ligands. Investigationsof the related intermediate-spin forms of these complexes for which S = 3/2 afford structures ofcomposition [Mn{η3-(Me3Si)2C3H3}2{η1-(Me3Si)2C3H3) }]- (1-IS) and [Mn(η3-C3H5)2(η1-C3H5)]- (2-IS), in which one of the allyl ligands has slipped from an η1- to η3-bonding mode with a concomitantdecrease in the manganese−carbon distances, and which again reveals essentially no dependency ofhapticity upon the spatial requirements of the silyl substituents. The relative energies of 1-HS, 1-IS, andthe low-spin state 1-LS (for which S = 1/2) at both the BP86/AE1 and B3LYP/AE1 levels of theoryrevealed 1-HS and 1-LS to be the most and least stable forms of 1, respectively, with the energy of 1-LSbeing prohibitively high at each level of theory. A reinvestigation of the experimental magneticsusceptibility of the ion-separated species [Li(thf)4][Mn{η3-(Me3Si)2C3H3}{η1-(Me3Si)2C3H3)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.
- Density functional theory calculations on the tris-(allyl)manganese(II) anions [Mn{ηx-(Me3Si)2C3H3}3]- and [Mn(ηx-C3H5)3]- (x = 1 or 3) with spin, S, equal to 5/2, 3/2, and 1/2 provide insight into the mixed ligand hapticities, revealing that substituents have no major structural influence and that S, hapticity, and the nature of the manganese−carbon bonds are interdependent. Prediction of a stable S = 3/2 state prompted a reinvestigation of the experimental susceptibility data of [Li(thf)4][Mn{η3-(Me3Si)2C3H3}{η1-(Me3Si)2C3H3)2}], the analysis suggesting the existence of such a spin state.
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