Further studies of metal-metal bonded oligomers of rhodium(I) isocyanide complexes. Crystal structure analysis of octakis(phenyl isocyanide)dirhodium bis(tetraphenylborate)

Abstract

The room temperature absorption spectra of [Rh(CNR)₄]⁺ (R = Ph, i-Pr, cyclohexyl, t-Bu, vinyl) in solution do not follow Beer's law. This behavior has been attributed to oligomerization of [Rh(CNR)₄]⁺ units to form species of the type [Rhₙ(CNR)₄ₙ]ⁿ⁺. Band maxima attributable to oligomers are as follows: R = Ph, 568 nm (n = 2), 727 nm (n = 3), in acetonitrile solution; R = t-Bu, 490 nm (n = 2), 622 nm (n = 3), in aqueous solution; R = i-Pr, 495 nm (n = 2), 610 nm (n = 3) in aqueous solution; R = cyclohexyl, 516 nm (n = 2), in acetonitrile solution; R = vinyl, 555 nm (n = 2), 715 nm (n = 3), 962 nm (n = 4), in aqueous solution. The molar extinction coefficients (ϵₙ) and formation constants Kₙ₋₁ have been obtained for R = Ph in acetonitrile solution and R = t-Bu in aqueous solution. Parameter values are as follows: for R = Ph, K₁ = 35 (15) M⁻¹, ϵ₂ = 1.05 (20) x 10⁴, ϵ₃K₂ = 1.83 (40) x 10⁵ M⁻¹, for R = t-Bu, K₁ = 250m (125) M⁻¹, ϵ₂ = 1.69 (34) x 10⁴. The x-ray crystal structure of Rh(CNPh)₄BPh₄ has been completed (final R = 00.057). The compound crystallizes in the Pbcn space group (a = 23.80 (1), b = 19.23 (1), c = 19.08 (1) Å) with four discrete cationic [Rh₂(CNR)₈]²⁺ units and eight BPh⁻⁴ anions. The dimeric cation has idealized D_(4d) symmetry; the two [Rh(CNR)₄]⁺ units are bonded face to face so as to give a staggered configuration of ligands. The Rh-Rh distance is 3.193 Å. The electronic absorption spectra of D_(4d) [Rh₂(CNR)₈]²⁺ and assumed D₄ₕ [Rh₃(CNR)₁₂]³⁺ complexes are interpreted in terms of the interactions expected between the occupied a_(1g)d_(z²) and unoccupied a_(2u)[p_z, π*(CNR)] monomer orbitals. The lowest band in each of the [Rh₂(CNR)₈]²⁺ complexes is assigned to the allowed 1 b₂ → 2a₁ transition. In the spectra of [Rh₃(CNR)₁₂]³⁺ complexes, the lowest band is attributed to 2a_(1g) → 2a_(2u).

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