Formation of the Unprecedented [Ge₉Ga–GaGe₉]⁴⁻ Cluster Anion through Nido-[Ge₉]⁴⁻ Cluster Extension with GaCp*

From a reaction of K₄Ge₉, GaCp*, and [18]crown-6 in ethylenediamine, the new compound [K([18]crown-6)]₄[Ga₂Ge₁₈]·4en has been obtained, containing the novel [Ge₉Ga–GaGe₉]⁴⁻ cluster anion. The compound crystallizes in space group P (Formula presented.) (no. 2) with a = 12.103(3) Å, b = 12.863(3) Å, c = 17.369(3) Å, α = 82.48(2)°, β = 81.51(2)°, γ = 76.35(2)°, V = 2585.7(11) Ǻ³, and Z = 1. From electrospray ionization mass spectrometry, the main peak indicates a [GaGe₉]⁻ group, that is, one half of the cluster anion. Quantum chemical calculations corroborate the experimental observations, confirming the location of the Ga atoms as bridging units within the dimeric 20-atom cluster. Alternative structural arrangements with Ga atoms in different positions are significantly less favorable in energy. The paramagnetic monomer [GaGe₉]²⁻ is energetically disfavored relative to dimer formation. The [Ge₉Ga–GaGe₉]⁴⁻ is only the second example of a germanium cluster dimer featuring a homoatomic bond between two Ga atoms with oxidation state +2, following the previously reported [Ge₉Zn–ZnGe₉]⁴⁻ comprising Zn with formal oxidation state +1.