Experimental measure of metal–alkynyl electronic structure interactions by photoelectron spectroscopy: (η5-C5H5)Ru(CO)2C CMe and [(η5-C5H5)Ru(CO)2]2(μ-C C)

Abstract

The gas-phase He I and He II photoelectron spectra of the propynylruthenium molecule CpRu(CO) 2 C≡CMe (Cp = η 5-C 5 H 5) and the ethynediyldiruthenium molecule [CpRu(CO) 2 ] 2 (µ-C≡C) are compared with the spectrum of CpRu(CO) 2 Cl to experimentally determine electronic structure interactions of the alkynyl ligands with the metal. The spectra indicate that the interaction between the filled metal-dπ and filled alkynyl-π orbitals dominates the metal-alkynyl π electronic structure, mirroring previously characterized CpFe(CO) 2 alkynyls. All valence ionizations of the Ru molecules are stabilized with respect to similar Fe compounds, contrary to the common expectation of lower ionization energies with atomic substitution down a column of the periodic table. Ab initio electronic structure calculations suggest that this stabilization traces to the greater inherent electronic relaxation energy associated with removal of Fe 3d electrons compared to removal of Ru 4d electrons. Destabilization of the first two ionization bands of the diruthenium molecule are a result of filled-filled interactions between alkynyl π-bonds with the symmetric combination of metal-metal-dπ orbitals, showing electronic communication between the metals through the alkynyl bridge. From the photoelectron spectrum, this communication was calculated to have a minimum electron-transfer integral of 0.56 eV. The stabilization of the antisymmetric combination of the metal-metal-dπ orbitals gives a direct and unique experimental measure of the interaction with the alkynyl π* orbitals. The stabilization caused by the alkynyl π* orbitals was found to be approximately one-third of the destabilization caused by the filled-filled interaction with the alkynyl π-bonds and about one-fourth to one-third the stabilization provided by back-bonding to a carbonyl ligand.

Rise of Conjugated Poly-ynes and Poly(Metalla-ynes): From Design Through Synthesis to Structure–Property Relationships and Applications

“…They concluded that there is a mixing of ligand and metal orbitals leading to extended conjugation through the metal sites. They also showed that a band gap difference (Δ E g ) of 0.3–0.7 eV exists between the square planar Pt­(II) poly-ynes and corresponding model complexes. , Lichtenberger and co-workers carried out ultraweak photon emission (UPE) spectroscopic studies to understand the electronic interactions between Ru­(II) and alkynyl ligands. They found that in these complexes too, metal-alkynyl π electronic structure is mainly dominated by the interaction between the filled metal- d π and filled alkynyl-π orbitals.…”

Computational Studies of Relative Stabilities of Low-Spin d6cis- andtrans-[M(en)2X2]+Complexes (M = Co, Rh, Ir): Steric and Electronic Effects in the Context of the StructuralTransInfluence

“…Literature data indicate that BCl 2 – , and SiCl 3 – , , while being much poorer acceptors than CO, nonetheless are modest π acceptor ligands. Indeed, Lichtenberger et al argued that SiCl 3 – is a better π acceptor than CN – and CCH – , based on photoelectron spectroscopic studies of CpM­(CO) 2 (X) complexes (M = Fe, Ru) .…”

“…Literature data indicate that BCl 2 −45,46 and SiCl 3 − , 47,48 while being much poorer acceptors than CO, nonetheless are modest π acceptor ligands. Indeed, Lichtenberger et al argued that SiCl 3…”

π Acceptor Abilities of Anionic Ligands: Comparisons Involving Anionic Ligands Incorporated into Linear d10 [(NH3)Pd(A)]−, Square Planar d8 [(NN2)Ru(A)]−, and Octahedral d6 [(AsN4)Tc(A)]− Complexes

“…The effects of π backbonding appear in aspects of transition-metal coordination chemistry ranging from evaluation of orbital gaps and the spectrochemical series to the thermodynamic trans influence and the kinetic trans effect. − A textbook example involves the trend in M­(CO) 6 q complexes where the extent of backbonding correlates with the carbon–oxygen vibrational stretching frequencies. Experimentally, evaluating relative amounts of π acceptor capacity for different ligand types has proved challenging, mostly because it is difficult to separate σ-bonding and π-bonding effects. − Carbonyl carbon–oxygen vibrational stretching frequencies have been used to assess the π backbonding capacities of other ligands bound to the same metal, but care must be taken in selecting ligands, metals, and geometries to obtain trustworthy results. − Lichtenberger and coworkers argued that gas-phase photoelectron spectroscopy is the only experimental technique that can quantitatively separate ligand π-bonding effects from σ-bonding and charge potential effects. , Using this technique, they found that CCH – acts as nearly a pure donor toward d 6 CpM­(CO) 2 (M = Fe and Ru) fragments rather than as an acceptor and that SiCl 3 – acts as a modest acceptor in these systems. Recently, Roithová, Rasika Dias, and coworkers used gas-phase bond dissociation to show that alkynes bind coinage metal complexes more strongly than alkenes do, a result they trace to greater backbonding in the former …”

“…7−13 Lichtenberger and coworkers argued that gas-phase photoelectron spectroscopy is the only experimental technique that can quantitatively separate ligand π-bonding effects from σ-bonding and charge potential effects. 14,15 Using this technique, they found that CCH − acts as nearly a pure donor toward d 6 CpM(CO) 2 (M = Fe and Ru) fragments rather than as an acceptor and that SiCl 3 − acts as a modest acceptor in these systems. Recently, Roithova, Rasika Dias, and coworkers used gas-phase bond dissociation to show that alkynes bind coinage metal complexes more strongly than alkenes do, a result they trace to greater backbonding in the former.…”