Further studies of metal-metal bonded oligomers of rhodium(I) isocyanide complexes. Crystal structure analysis of [Rh2(CNPh)8](BPh4)2

The room-temperature electronic 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 [Rh_n(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 (ε_n) and formation constants K_n-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) × 10⁴, ε₃K₂ = 1.83 (40) × 10⁵ M^-1; for R = t-Bu, K₁ = 250 (125) M^-1, ε₂ = 1.69 (34) × 10⁴. The x-ray crystal structure of Rh(CNPh)₄BPh₄ has been completed (final R = 0.057). 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(CNPh)₈]²⁺ units and eight BPh₄^- anions. The dimeric cation has idealized D_4d symmetry; the two [Rh(CNPh)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 D_4h [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 1b₂ → 2a₁ transition. In the spectra of [Rh₃(CNR)₁₂]³⁺ complexes, the lowest band is attributed to 2a_1g → 2a_2u.