A dichromium(II) bis(η⁸-pentalene) double-sandwich complex with a spin equilibrium: Synthetic, structural, magnetic, and theoretical studies

The new bis(pentalene) complex Cr₂(η⁵: η⁵-C₈H₄ ^1,4SiiPr 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 = 1/2 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.