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 [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)X104, ε3K2=1.83 (40)X105 M-1; for R=t-Bu, K1=250 (125) M-1, ε2=1.69 (34)X104. 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 D4d 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 D4d [Rh2(CNR)8]2+ and assumed D4h [Rh3(CNR)12]3+ complexes are interpreted in terms of the interactions expected between the occupied a1g(dz2) and unoccupied a2u[pz, π*(CNR)] monomer orbitals. The lowest band in each of the [Rh2(CNR)g]2+ complexes is assigned to the allowed 1b2→2aj transition. In the spectra of [Rh3(CNR)12]3+ complexes, the lowest band is attributed to 2a1g' →2a2u.