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Issue 24, 1994
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Interpretation of the electronic spectra of the confacial bioctahedral nonahalides [Ru2Cl9]3– and [Ru2Br9]3– using the SCF-Xα-SW method: location of the σ→σ* transition

Abstract

Considerable progress has been made in assigning the UV/VIS/near-IR electronic spectra (5000–50 000 cm–1) of [Ru2Cl9]3– and [Ru2Br9]3–, with the aid of SCF-Xα-SW calculations. A conceptual framework within which band energies and relative intensities can be discussed was constructed by reference to the corresponding [MX6]z monomers. First, an empirical correction (δc.t.) of 7500 cm–1 has been established for Xα-computed energies of frankly charge-transfer (c.t.) transitions, based on the discovery that the calculated X → M charge-transfer (x.m.c.t.) transitions are faithfully linearly correlated (unity gradient) with observed band energies for the congeneric t2g5 complexes [RuCl6]3–, [RuBr6]3–, [IrCl6]2–, and [IrBr6]2–. Secondly, the calculated oscillator strengths for c.t. transitions of monomeric hexahalides successfully model observed band intensities, and can be understood in terms of the mutual overlap of ligand-based components of the donor and acceptor orbitals. For binuclear [Ru2Cl9]3– and [Ru2Br9]3– the intense absorption bands above 30 000 cm–1 are readily assigned to x.m.c.t. transitions to the {7e″, 9e′} levels, derived from the single-ion eg orbitals. In contrast, the anticipated c.t. to the t2g-derived levels (i.e. to unfilled 5a2″) loses intensity in the trigonal field of the confacial complexes, and the prominent near-UV/VIS features are assigned instead to transitions within the metal-metal orbital manifold. In particular, the binuclear σ→σ* transition is located at 22 500 cm–1 for [Ru2Cl9]3– and 20 000 cm–1 for [Ru2Br9]3–, well above the calculated values. The discrepancy between observed and calculated σ→σ* transition energies arises through the neglect of electron-correlation effects in the Xα calculations. The implied value of the two-electron exchange term K is about 7800 cm–1 for both complexes. Thus, two major empirical adjustments (δc.t. and K) are required in the present analysis due to the limitations of the Xα method, and the status of these measures is examined.

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Article type: Paper
DOI: 10.1039/DT9940003759
Citation: J. Chem. Soc., Dalton Trans., 1994,0, 3759-3767
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    Interpretation of the electronic spectra of the confacial bioctahedral nonahalides [Ru2Cl9]3– and [Ru2Br9]3– using the SCF-Xα-SW method: location of the σ→σ* transition

    G. A. Heath and J. E. McGrady, J. Chem. Soc., Dalton Trans., 1994, 0, 3759
    DOI: 10.1039/DT9940003759

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