Large Ligand Folding Distortion in an Oxomolybdenum Donor–Acceptor Complex

Interligand charge transfer is examined in the novel metallo-dithiolene complex MoO­(SPh)₂(^<i>i</i>Pr₂Dt⁰) (where ^<i>i</i>Pr₂Dt⁰ = <i>N</i>,<i>N</i>′-isopropyl-piperazine-2,3-dithione). The title complex displays a remarkable 70° “envelope”-type fold of the five-membered dithiolene ring, which is bent upward toward the terminal oxo ligand. A combination of electronic absorption and resonance Raman spectroscopies have been used to probe the basic electronic structure responsible for the large fold-angle distortion. The intense charge transfer transition observed at ∼18 000 cm^–1 is assigned as a thiolate → dithione ligand-to-ligand charge transfer (LL′CT) transition that also possesses Mo­(IV) → dithione charge transfer character. Strong orbital mixing between occupied and virtual orbitals with Mo­(<i>x</i>²–<i>y</i>²) orbital character is derived from a strong pseudo Jahn–Teller effect, which drives the large fold-angle distortion to yield a double-well potential in the electronic ground state