[Gd4(C2)](Cl,I)6, an Interstitially Stabilized Heteroleptic Gadolinium Sesquihalide

[Gd₄(C₂)](Cl, I)₆ is obtained from CsI, Gd, GdCl₃ and C₂I₄ in sealed niobium containers at 1000/800°C as black, shiny needles. The crystal structure (tetragonal, P4/mbm, Z = 2, a = 1347.5(1), c = 1212.5(1) pm) is similar to that of Na[Mo₄]O₆ and [Sc₄B]Cl₆. It may be regarded as being built from octahedra sharing common trans edges running in the [001] direction. The octahedra contain C₂ units as interstitials. Every third octahedron contains a disordered C₂ unit perpendicular to those in the two neighboring [Gd₆(C₂)] octahedra and is therefore compressed in the direction of the (pseudo) C₄ axis. Calculations of the electronic structure of an “empty” [Gd₄]Cl₆ structure reveals a total of 13 electrons necessary to occupy all metal-metal bonding states. The incorporation of a carbon dimer substantially alters the bonding conditions for [Gd₄(C₂)]X₆ (X = Cl, I). The formal charge of ?6 of the C₂ unit is significantly reduced as π_g states split up, Gd? Gd and Gd? C bonding states are occupied and bonding d orbitals combine to form the lowest unoccupied energy states.