Heterobimetallic Uranium-Nickel/Palladium/Platinum Complexes of Phosphinoaryl Oxide Ligands: A Theoretical Probe for Metal-Metal Bonding and Electronic Spectroscopy.

Heterobimetallic uranium-transition metal (U-TM) complexes have abundant active centers (two metals and several ancillary ligands with various donor atoms) and possible metal-metal bonding interaction, leading to diversified electronic structures and rather complicated electronic transition types. In this regard, a comprehensive and systematic theoretical study is highly desired although challenging. In the work, density functional theory (DFT) was utilized to examine a series of uranium-group 10 metal complexes supported by bidentate phosphinoaryl oxide ligands (labeled as L). TM (Ni, Pd, and Pt), uranium oxidation state (IV and III) and axial donor (I, Br, Cl, F, Me3SiO, and vacant) were varied. Calculations demonstrate an intrinsic TM → U dative bond. The order of bond strength of U-Ni > U-Pt > U-Pd is suggested by the formal shortness ratios, quantum theory of atoms in molecule (QTAIM) data, interaction energy ( Eint), and bond orders calculated at various levels of theory. It is further evidenced by relativistic effects of heavy metal, natural orbital population and electronic spectroscopy. Regarding U-Ni complexes with different axial donors, metal-metal distances are found to be linearly correlated with QTAIM data/ Eint/bond orders. Experimental UV-vis-NIR spectra were well reproduced by time-dependent DFT calculations. Complicated visible-light absorption bands, whose understanding remains unclear for many experimentally known heterobimetallic complexes, were rationalized in the work, along with NIR bands assigned as 5f → 5f transitions.