Search for Osme Bonds with π Systems as Electron Donors

Abstract

The Osme bond is defined as pairing a Group 8 metal atom as an electron acceptor in a noncovalent interaction with a nucleophile. DFT calculations with the ωB97XD functional consider MO₄ (M = Ru, Os) as the Lewis acid, paired with a series of π electron donors C₂H₂, C₂H₄, C₆H₆, C₄H₅N, C₄H₄O, and C₄H₄S. The calculations establish interaction energies in the range between 9.5 and 26.4 kJ/mol. Os engages in stronger interactions than does Ru, and those involving more extensive π-systems within the aromatic rings form stronger bonds than do the smaller ethylene and acetylene. Extensive analysis questions the existence of a true Osme bond, as the bonding chiefly involves interactions with the three O atoms of MO₄ that lie closest to the π-system, via π(C-C)→σ*(M-O) transfers. These interactions are supplemented by back donation from M-O bonds to the π*(CC) antibonding orbitals of the π-systems. Dispersion makes a large contribution to these interactions, higher than electrostatics and much greater than induction.

Keywords: AIM; NBO; SAPT; back donation; σ-hole.

Figure 1

Molecular electrostatic potential (MEP) maps of monomers. Red and blue colors, respectively, indicate the most positive and negative values. Numerical values of extrema in a.u.

Figure 2

AIM molecular diagrams showing intermolecular bond paths; small red balls indicate position of bond critical point.

Figure 3

Examples of NBOs involved in charge transfer within the OsO₄-C₄H₅N complex. (a) E₁: π(CC)→σ*(OsO_adj), (b) E₂: π(CC)→σ*(OsO_opp), (c) E₃: (OsO)→π*(CC), (d) E₄: O_LP→π*(CC).

Figure 4

Frontier MOs of acetylene and RuO₄. Occupied orbitals lie below the broken black line, and virtual orbitals above. Purple and green regions indicate opposite signs within each wavefunction. Numbers refer to difference in orbital energies in a.u.

Figure 5

Reduced density gradient visualization of the bonding between C₄H₅N and (a) RuO₄ and (b) OsO₄. RDG contour is 0.5 a.u., where blue and red colors represent −0.1 and +0.1 a.u. for ρ·sign(λ₂).

Figure 6

Electron density difference diagram representing the shift of density resulting from the complexation between OsO₄ and C₆H₆. Purple and green colors indicate gain and loss, respectively, by 0.0002 a.u.