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.