INTERPRETATION OF THE ELECTRONIC-SPECTRA OF THE CONFACIAL BIOCTAHEDRAL NONAHALIDES [RU2CL9](3-) AND [RU2BR9](3-) USING THE SCF-X-ALPHA-SW METHOD - LOCATION OF THE SIGMA-]SIGMA-ASTERISK-TRANSITION

Considerable progress has been-made in assigning the V/VIS/near-IR electronic spectra(5000-50 000 cm(-1)) of [Ru2Cl9](3-) and [Ru2Br9](3-), with the aid of SCF-X alpha-SW ca(culations. A conceptual framework within which band energies and relative intensities can be discussed was constructed by reference to the corresponding [MX(6)](z-) monomers. First, an empirical correction (delta(c.t.)) of 7500 cm(-1) has been established for X alpha-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 t(2g)(5) complexes [RUCl(6)](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 e(g) orbitals. In contrast, the anticipated c.t. to the t(2g)-derived levels (i.e. to unfilled 5a(2)'') 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 sigma-->sigma* transition is located at 22 500 cm(-1) for [Ru2Cl9](3-) acid 20 000 cm(-1) for [Ru2Br9](3-), well above the calculated values. The discrepancy between observed and calculated sigma-->sigma* transition energies arises through the neglect of electron-correlation effects in the X alpha calculations. The implied value of the two-electron exchange term K is about 7800 cm(-1) for both complexes. Thus, two major empirical adjustments (delta(c.t.) and K) are required in the present analysis due to the limitations of the X alpha method, and the status of these measures is examined.