Interplay between Structural Reorganization and Metal – Ligand Covalency in the Distinct Excited States of [M(diimine)(dithiolate)] Complexes

This work identifies the impact of covalent metal–ligand interactions on the excited states of planar [M(diimine)(dithiolate)] complexes (M = Pt, Ni) with bipyridyl (bpy) and benzene-1,2-dithiolate (bdt) ligands. The long-lived excited states of the Pt(II) complexes have long been surmised to have metal-and-dithiolate ligand → diimine ligand charge transfer (MLL’CT) character due to disproportionately strong metal–dithiolate orbital mixing, whereas the nature of the shorter-lived excited states of the nonluminescent Ni(II) complexes has not been investigated. Time-resolved Pt 2p (L3-edge) X-ray absorption near-edge structure (XANES) spectroscopy is presented herein to definitively prove that strong orbital overlap between the metal center and dithiolate ligand yields the MLL’CT character of the long-lived excited state in [Pt(bpy)(bdt)]. For [Ni(bpy)(bdt)], ultrafast transient Ni 1s (K-edge) XANES is used to resolve the electronic and structural relaxation dynamics following MLL’CT excitation. Combined with electronic structure calculations, the results show that the low-lying Ni 3dyz orbital in [Ni(bpy)(bdt)] facilitates subpicosecond formation of a metastable triplet excited state with ligand field state symmetry (3LF). Tetrahedral structural relaxation dynamically alters the differential Ni–diimine and Ni–dithiolate covalencies and thereby modulates the resulting excited state charge distribution. The Ni-and-bpy → bdt charge transfer character of the fully relaxed 3LF state is found to reverse the directionality of the ligand-to-ligand charge transfer established for the 3MLL’CT state of the Pt congener. Altogether, these results demonstrate that inequivalent metal–ligand covalencies in mixed-ligand donor–acceptor complexes could be applied to control excited state charge distributions within photoactive transition metal complexes.