Tunable Control of Magnetic Properties in Transition Metal Indenyl Complexes Timothy P. Hanusa, Department of Chemistry, Vanderbilt University, Nashville, TN Magnetism with a Twist Magnetic properties of bis(indenyl) complexes of chromium(II) vary with the relative orientation of the indenyl ligands, a reflection of symmetry (mis)-matching between the ligand and metal d orbitals. We have obtained spectacular evidence of this in the crystal structure of (1,4,7-Me3C9H4)2Cr, which contains molecules in both eclipsed and staggered conformations. Only the staggered form displays significant spin-crossover behavior. A series of substituted bis(indenyl)chromium(II) complexes has been prepared in which methyl groups on the six-membered benzo ring appear to be more effective in stabilizing a low-spin ground state than those on the five-membered ring. The tendency to exist as a low-spin complex is approximately as follows: (1,3-(SiMe3)2C9H4)2Mn > ~ > > Mn(III) ~ Cr(II) ? One goal of this research is to incorporate spin crossover compounds into charge-transfer molecular magnets, and create new classes of magnetically active materials for potential data storage applications. A step toward this has been taken in the isolation of the first bis(indenyl)manganese(II) complexes. When oxidized to their cations, they will be examined for spin-crossover behavior parallel to that of their isoelectronic neutral Cr(II) counterparts.