[GD-4(C-2)](CL,I)(6), AN INTERSTITIALLY STABILIZED HETEROLEPTIC GADOLINIUM SESQUIHALIDE

[Gd-4(C-2)](Cl, I)(6) is obtained from CsI, Gd, GdCl3 and C2I4 in seal ed niobium containers at 1000/800 degrees C as black, shiny needles. T he crystal structure (tetragonal, P4/mbm, Z = 2, a = 1347.5(1), c = 12 12.5(1)pm) is similar to that of Na[Mo-4]O-6 and [Sc4B]Cl-6. It may be regarded as being built from octahedra sharing common trans edges run ning in the [001] direction. The octahedra contain C-2 units as inters titials. Every third octahedron contains a disordered C-2 unit perpend icular to those in the two neighboring [Gd-6(C-2)] octahedra and is th erefore compressed in the direction of the (Pseudo) C-4 axis. Calculat ions of the electronic structure of an ''empty'' [Gd-4]Cl-6 structure reveals a total of 13 electrons necessary to occupy all metal-metal bo nding states. The incorporation of a carbon dimer substantially alters the bonding conditions for [Gd-4(C-2)]X(6) (X = C1, I). The formal ch arge of -6 of the C-2 unit is significantly reduced as pi(g) states sp lit up, Gd-Gd and Gd-C bonding states are occupied and bonding d(x2-y2 ) orbitals combine to form the lowest unoccupied energy states.