The electronic structures of the diruthenium compounds Ru(ap)Cl (1, ap = 2-anilinopyridinate) and Ru(ap)OTf (2) were investigated with UV-vis, resonance Raman, and magnetic circular dichroism (MCD) spectroscopies; SQUID magnetometry; and density functional theory (DFT) calculations. Both compounds have quartet spin ground states with large axial zero-field splitting of ∼60 cm that is characteristic of Ru compounds having a (π*, δ*) electron configuration and a Ru-Ru bond order of ∼2.5. Two major visible absorption features are observed at ∼770 and 430 nm in the electronic spectra, the assignments of which have previously been ambiguous. Both bands have significant charge-transfer character with some contributions from d → d transitions. MCD spectra were measured to enable the identification of d → d transitions that are not easily observable by UV-vis spectroscopy. In this way, we are able to identify bands due to δ → δ* and δ → π* transitions at ∼16 100 and 11 200-12 300 cm, respectively, the latter band being sensitive to the π-donating character of the axial ligand. The Ru-Ru stretches are coupled with pyridine rocking motions and give rise to observed resonance Raman peaks at ∼350 and 420 cm, respectively.

We report the transmetalation of hydrocarbyl fragments (Me, Bn, Ph) from a variety of organometallic complexes relevant to C–H activation (Ir, Rh, W, Mo) to Pt­(II) electrophiles. The scope of suitable hydrocarbyl donors is remarkable in that three different classes of organometallics with widely varying reactivity all undergo the same general reaction with Pt­(II) electrophiles. A competitive substituent effect experiment reveals faster transmetalation of more electron-rich hydrocarbyl groups. This study suggests that transmetalation could provide a viable path for catalytic functionalization of stable complexes resulting from C–H bond activation and other processes.

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Diruthenium paddlewheel complexes supported by electron-rich anilinopyridinate (Xap) ligands were synthesized in the course of the first in-depth structural and spectroscopic interrogation of monocationic [Ru2(Xap)4Cl]+ species in the Ru2 6+ oxidation state. Despite paramagnetism of the compounds, 1H NMR spectroscopy proved highly informative for determining the isomerism of the Ru2 5+ and Ru2 6+ compounds. While most compounds are found to have the polar (4,0) geometry, with all four Xap ligands in the same orientation, some synthetic procedures resulted in a mixture of (4,0) and (3,1) isomers, most notably in the case of the parent compound Ru2(ap)4Cl. The isomerism of this compound has been overlooked in previous reports. Electrochemical studies demonstrate that oxidation potentials can be tuned by the installation of electron donating groups to the ligands, increasing accessibility of the Ru2 6+ oxidation state. The resulting Ru2 6+ monocations were found to have the expected (π*)2 ground state, and an in-depth study of the electronic transitions by Vis/NIR absorption and MCD spectroscopies with the aid of TD-DFT allowed for the assignment of the electronic spectra. The empty δ* orbital is the major acceptor orbital for the most prominent electronic transitions. Both Ru2 5+ and Ru2 6+ compounds were studied by Ru K-edge X-ray absorption spectroscopy; however, the rising edge energy is insensitive to redox changes in the compounds due to the broad line shape observed for 4d transition metal K-edges. DFT calculations indicate the presence of ligand orbitals at the frontier level, suggesting that further oxidation beyond Ru2 6+ will be ligand-centered rather than metal-centered.

Understanding the fundamental properties governing metal− metal interactions is crucial to understanding the electronic structure and thereby applications of multimetallic systems in catalysis, material science, and magnetism. One such property that is relatively underexplored within multimetallic systems is metal−metal bond polarity, parameterized by the electronegativities (χ) of the metal atoms involved in the bond. In heterobimetallic systems, metal−metal bond polarity is a function of the donor−acceptor (Δχ) interactions of the two bonded metal atoms, with electropositive early transition metals acting as electron acceptors and electronegative late transition metals acting as electron donors. We show in this work, through the preparation and systematic study of a series of Mo 2 M(dpa) 4 (OTf) 2 (M = Cr, Mn, Fe, Co, and Ni; dpa = 2,2′dipyridylamide; OTf = trifluoromethanesulfonate) heterometallic extended metal atom chain (HEMAC) complexes that this expected trend in χ can be reversed. Physical characterization via single-crystal X-ray diffraction, magnetometry, and spectroscopic methods as well as electronic structure calculations supports the presence of a σ symmetry 3c/3e − bond that is delocalized across the entire metal-atom chain and forms the basis of the heterometallic Mo 2 −M interaction. The delocalized 3c/3e − interaction is discussed within the context of the analogous 3c/3e − π bonding in the vinoxy radical, CH 2 CHO. The vinoxy comparison establishes three predictions for the σ symmetry 3c/3e − bond in HEMACS: (1) an umpolung effect that causes the Mo−M interactions to become more covalent as Δχ increases, (2) distortion of the σ bonding and non-bonding orbitals to emphasize Mo−M bonding and de-emphasize Mo−Mo bonding, and (3) an increase in Mo spin population with increasing Mo−M covalency. In agreement with these predictions, we find that the Mo 2 •••M covalency increases with increasing Δχ of the Mo and M atoms (Δχ Mo−M increases as M = Cr < Mn < Fe < Co < Ni), an umpolung of the trend predicted in the absence of σ delocalization. We attribute the observed trend in covalency to the decreased energic differential (ΔE) between the heterometal d z 2 orbital and the σ bonding molecular orbital of the Mo 2 quadruple bond, which serves as an energetically stable, "ligand"-like electron-pair donor to the heterometal ion acceptor. As M is changed from Cr to Ni, the σ bonding and nonbonding orbitals do indeed distort as anticipated, and the spin population of the outer Mo group is increased by at least a factor of 2. These findings provide a predictive framework for multimetallic compounds and advance the current understanding of the electronic structures of molecular heteromultimetallic systems, which can be extrapolated to applications in the context of mixed-metal surface catalysis and multimetallic proteins.