A Dichromium(II) Bis(η 8 -pentalene) Double-Sandwich Complex with a Spin Equilibrium: Synthetic, Structural, Magnetic, and Theoretical Studies

The new bis(pentalene) complex Cr 2 (η 5:η 5 -C 8 H 4 1,4-Si i Pr 3 ) 2 has been synthesized and characterized; it is found to exhibit paramagnetism at room temperature, and solid-state magnetic studies show that the dimer is best modeled as containing a pair of antiferromagnetically interacting S = ½ centers with the separation between the singlet ground state and triplet excited state being 2.23 kJ mol −1 . Structural data show a Cr−Cr distance of 2.2514(15) Å, consistent with a strong metal−metal interaction. The bonding has been further investigated by density functional, hybrid, and CASPT2 methods. The metal−metal interaction is best described by a double bond with each metal having an 18-electron count. Theory predicts the singlet and triplet states to lie close in energy but puts the triplet state at a slightly lower energy than the singlet. The energy difference predicted by CASPT2 is closest to the experimental value.