Magnetic circular dichroism of metal to ligand charge transfer spectra of some selected d⁶ and d¹⁰ transition metal complexes
Chastain, Steven K.
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Electronic absorption and magnetic circular dichroisn (MCD) spectral measurements are reported for the octahedral nd⁶ carbonyl and cyano complexes M(CO)₆ (M = Cr, Mo, W) and K₄[M(CN)₆] (M = Fe, Ru, Os), the linear nd¹⁰ complexes [M(CNC₂H₅)₂]ClO₄ (M = Ag, Au, Hg), [Au(P(OR)₃)₂]ClO₄ (R = CH₃, C₂H₅), [Au(CNCH₃)CN], and the tetrahedral 3d¹⁰ complexes K₃[Cu(CN)₄], and [Cu(CNC₂H₅)₄]ClO₄. The intense absorptions observed for the M(CO)₆ and [M(CN)₆]⁴⁻ complexes are characterized as metal to ligand charge transfer (MLCT) from the occupied metal d orbitals to the empty π* ligand orbitals (t[sub 2g] → t[sub lu]* and t[sub 2u]*). The spectral pattern observed in the MCD for the two lowest allowed bands of the M(CO)₆ is similar and consists of a positive B̲ term for the band near 3.5 μm⁻¹ and a negative B̲ term for the band near 4.4 μm⁻¹. The difference between the MCD for the lowest energy MLCT between M(CO)₆ and [M(CN)₆]⁴⁻ complexes is discussed in terms of relative p orbital participation in the empty t[sub lu]* ligand orbital. The rich absorption and MCD spectra for [Au(CNC₂H₅)₂]C1O₄ are interpreted in terms of MLCT excitations (d(σ⁺[sub g], δ [sub g], π[sub g]) → π*). A detailed interpretation of [Au(CNC₂H₅)₂]C1O₄ spectra is given based on an energy level scheme which includes spin-orbit coupling. Spectral assignments for [Au(CNC₂H₅)₂]⁺ are compared to the similar absorption bands in the [Au(CN)₂]⁻, [Au(P(OR)₃)₂]⁺, and [Au(CNCH₃)CN] complexes. The results are also discussed in terms of d orbital participation in bonding with these π- acceptor ligands. The spectra for K₃[Cu(CN)₄] consists of two major absorption bands at 4.21 and 4.85 μm⁻¹. The MCD for these bands show two distinct positive A̲ terms for both absorption bands. These bands are assigned to a MLCT and ligand to ligand (π → π*) excitation, respectively. A comparison of these assignments with the spectra of [Cu(CNC₂H₅)₄]ClO₄ is also presented.