Figure 5: Molecular structure and bonding description of [In^I₄(phen)₆]⁴⁺ (7⁴⁺) as well as spin density distributions of potential precursors.

(a) Molecular structure of 7⁴⁺. Selected bond lengths are given in pm, angles in ° and the values for 6⁴⁺ are parenthesized: In1−In2=273.03(10) (275.93(14)), In1−In3=258.06(16) (259.65(12)), In1−In4=286.06(11) (275.93(14)), In2−In3=286.06(11) (280.83(14)), In3−In4=273.03(10) (278.56(14)), av. In1−N=231.2(9) (232.40(10)), av. In2−N=235.6(8) (236.33(10)), av. In3−N=231.2(9) (233.60(10)), av. In4−N=235.6(8) (236.10(10)) and ∢(In−In−In)=54.92(3)−65.11(3) (55.59(3)−63.16(4)). The [Al(OR^F)₄]^– anions and all of the hydrogen atoms were omitted for clarity. (b) A possible hypothetic description of the bonding situation in 6⁴⁺ and 7⁴⁺. The short bridging In1−In3 values could originate from interactions between the π*-orbitals of a dicationic, formally doubly bonded [(N-ligand)In=In(N-ligand)]²⁺ fragment (N-ligand=bipy, phen) and singly occupied orbitals of two [In(N-ligand)₂]⁺ complexes, thus resulting in two two-electron three-centre bonds (2e3c), partially, but not fully, populating the antibonding π*-orbital. The remaining small In=In double-bonding contribution could account for the observed short In−In separations in 6⁴⁺ and 7⁴⁺ and also the relatively long 2e3c In−In bonds to the upper and lower [In(N-ligand)₂]⁺ moieties. c) Calculated spin density distributions of triplet state fragments that could interact to form the observed cationic clusters (spin density cutoff at 0.010 a.u., B3LYP/SV(P) level). In the calculated [(bipy)In=In(bipy)]³⁺ fragment, the In−In distance is 291.4 pm and the average spin density on each indium atom 77% (a planar dicationic fragment did not converge, even if the conductor-like screening model (COSMO)⁶⁵ was switched on and the permittivity was set to infinite ɛ_r=∞). For triplet state of [In(bipy)₂]⁺, the spin density on the indium atom is 35%.