The rotational orientation of cyclopentadienyl rings usually has no effect on d-orbital energy levels and splitting in transition metal complexes. With related but less symmetrical carbocyclic ligands, however, the magnetic properties of the associated complexes can be altered by the alignment of the ligands. Examples of this effect are found in substituted organochromium(II) bis(indenyl) complexes. The monosubstituted compounds (1-RC(9)H(6))(2)Cr (R = t-Bu, SiMe(3)) are prepared from the substituted lithium indenides and CrCl(2) in THF; they are high-spin species with four unpaired electrons. Their spin state likely reflects that in the unknown monomeric (C(9)H(7))(2)Cr, which is calculated to have a high-spin (S = 2) ground state in the staggered configuration (180 degrees rotation angle). However, the analogous bis(indenyl) complexes containing t-Bu or SiMe(3) groups in both the 1 and 3 positions on the indenyl ligands ((1,3-R(2)C(9)H(5))(2)Cr) are low-spin compounds with two unpaired electrons. X-ray diffraction results indicate that [1-(t-Bu)C(9)H(6)](2)Cr exists in a staggered conformation, with Cr-C (av) = 2.32(4) A. In contrast, the average Cr-C distances in [1,3-(t-Bu or SiMe(3))(2)C(9)H(5)](2)Cr are 2.22(2) and 2.20(2) A, respectively, and the rings are in a gauche configuration, with rotation angles of 87 degrees. The indenyl conformations are sterically imposed by the bulk of the t-Bu and SiMe(3) substituents. The change from a staggered to a gauche indenyl orientation lowers the symmetry of a (C(9)H(7))(2)M complex and allows greater mixing of metal and ligand orbitals. Calculations indicate that previously nonbonding pi orbitals of the indenyl anion are able to interact with the chromium d orbitals, producing bonding and antibonding combinations. The latter remain unpopulated, and the resulting increase in the HOMO-LUMO gap forces the complexes to adopt a low-spin configuration. The possibility of using sterically imposed ligand rotation as a means of spin-state manipulation makes indenyl compounds a potentially rich source of magnetically adjustable molecules.

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Previous work on bis(indenyl)chromium(II) complexes substituted with bulky groups (i-Pr, t-Bu, SiMe 3 ) found that their spin state (S ) 1 or 2) depended on the symmetry of the molecules. Complexes with inversion symmetry (staggered rings) were high-spin; lower symmetry compounds with twisted (gauche) ligands were low spin. The present work explores the effect of methyl group substitution on the indenyl ligand, which leads to complexes possessing either staggered or eclipsed conformations. The monosubstituted compounds [(1 or 2)-MeC 9 H 6 ] 2 Cr are prepared from the substituted alkali metal indenides and CrCl 2 in THF. X-ray diffraction results indicate that (2-MeC 9 H 6 ) 2 Cr exists in a staggered conformation, with Cr-C (av) ) 2.308(7) Å, and is a high-spin species in the solid state and solution. In contrast, the monomeric (1-MeC 9 H 6 ) 2 Cr is eclipsed in the solid state, where it exhibits spin-crossover behavior over a wide temperature range; the average Cr-C distances vary with temperature, from 2.179(9) Å at 105 K to 2.262(10) Å at 298 K. An attempt to form (4-MeC 9 H 6 ) 2 Cr produced the dimeric, thermally unstable complex (η 5 -indenyl′) 2 (µ,η 3 -indenyl′)Cr 2 (µ-Cl) instead. Correlations between the structure and magnetic properties in bis(indenyl)chromium(II) complexes have been made with density functional theory calculations, which indicate that an eclipsed ligand conformation supports a high spin state, but not to the extent that the staggered form does.