Mixed crystal of bis-(ammonium/oxonium) tetra-aqua-μ 3 -fluorido-dodeca-kis-(μ 2 -tri-fluoro-acetato) octa-hedro -hexa-ytterbiate(III) tetra-hydrate, [(NH 4 ) 1- x (H 3 O) x ] 2 [Yb 6 F 8 (O 2 CCF 3 ) 12 (H 2 O) 4 ]·4H 2 O ( x = 1/4), containing a hexa-nuclear ytterbium(III) carboxyl-ate complex with face-capping fluoride ligands and comprising an unusual kind of substitutional disorder.

The reaction of ytterbium metal with ammonium tri-fluoro-acetate in liquid ammonia resulted in a green substance comprising a substantial amount of ytterbium(II) tri-fluoro-acetate that is a useful precursor for the oxidative synthesis of the new ytterbium(III) compound, [(NH 4 ) 1- x (H 3 O) x ] 2 [Yb 6 F 8 (O 2 CCF 3 ) 12 (H 2 O) 4 ]·4H 2 O ( x = 1/4), in aqueous tri-fluoro-acetic acid. This mixed ammonium/oxonium crystalline solid is the first example of a substance containing an octa-hedro -hexa-nuclear ytterbium(III) complex with μ 3 -face-capping fluorido ligands. The main structural features of its [Yb 6 F 8 ] core are non-bonding Yb⋯Yb distances and Yb-F bond lengths of 3.7576 (3)-3.9413 (5) and 2.2375 (17)-2.3509 (17) Å, respectively. Yb-O bond lengths involving the O atoms of O , O '-bridging carboxyl-ato ligands and vertex-substituting aqua ligands are in the ranges 2.23 (4)-2.329 (2) and 2.448 (2)-2.544 (3) Å, respectively. These bond lengths are in accordance with expectations, taking into account lanthanoid contraction. Inter-estingly, there is a significant ammonium versus oxonium ion site dependence, not only of the hydrate water mol-ecule positions within the solid's hydrogen-bonding framework, but also of the coordination sites of one carboxyl-ato and one aqua ligand in the hexa-nuclear complex.