Ca–Ca interaction in inverse sandwich Ca–C8H8–Ca

Abstract

The inverse sandwich Ca–C₈H₈–Ca is predicted to be an open-shell singlet state. Since the C₈H₈ ligand prevents the spin-up and spin-down electrons of different calcium atoms from forming Ca–Ca bonds, the spin-coupling electrons lead to a singlet diradical character. The metal–ligand interaction contributes to the stability of Ca–C₈H₈–Ca against dissociation and isomerization. For the coordination complex (DME)₃Ca–C₈H₈–Ca(DME)₃, the open-shell singlet state is unavailable, while the closed-shell singlet state with direct Ca–Ca bonds is more favorable, because dimethyl ether molecules could push the spin-paired electrons of different calcium atoms to migrate towards the direction of Ca–Ca bonding. For Ca–C₄H₄–Ca, the ground state is an open-shell singlet state, of which the diradical character is very similar to that of Ca–C₈H₈–Ca. For (DME)₃Ca–C₄H₄–Ca(DME)₃, the lowest energy is the triplet state.