BaSn 5 is the tin richest phase in the system Ba/Sn and is obtained by stoichiometric combination of the elements. The compound peritecticly decomposes under formation of BaSn 3 and a Sn±Ba melt at 430°C. The structure shows a novel structure motive in tin chemistry. Tin atoms are arranged in graphite-like layers (honeycombs). Two such layers form hexagonal prisms which are centered by Sn. Consequently the central tin atom has the unusual coordination number 12. The two-dimensional tin slabs which consist of two 3 6 and one 6 3 nets of Sn atoms are separated by 6 3 nets of Ba atoms with Ba above the center of each tin hexagon. The structure of BaSn 5 can be rationalized as a variante of AlB 2 and thus also of the superconducting MgB 2 .Temperature dependent magnetic susceptibility measurements show that BaSn 5 is superconducting with T c = 4.4 K.Reinvestigation of the magnetism of the Ba richer phase BaSn 3 reveals for this compound a T c of 2.4 K. LMTO band structure and density of states calculations verify the metallic behavior of BaSn 5 . The van Hove scenario of high-temperature cuprate superconductors is discussed for this`classical' intermetallic superconductor. An analysis of the electronic structure with the help of fat-band projections and the electron localization function (ELF) shows that the van Hove singularity in the DOS originates from non-bonding (lone) electron pairs in the intermetallic phase BaSn 5 . The role of lone pairs in intermetallic phases is discussed with respect to superconducting properties.

3: To a solution of 1 [14,15] (86 mg, 0.10 mmol) and oxalyl chloride (254 mg, 2.0 mmol) in dry CH,Cl, (10 mL) was added DMF (0.025 mL) through a silica gel filter, and the mixture was stirred at room temperature for 8 h. The reaction mixture was evaporated in vacuo to obtain the acid chloride (102 mg (m,8H),3.99(m,20H),3.63(m,8H),3.46(m,4H),2.79(m,4H),2.62(m,4H) 42.3, 41.4. 37.5, 36.9, 31.9, 30.5, 27.6 (ZC), 18.9, 14.0.The octaester (298 mg, 0.093 mmol) was added to "FA (3 mL) and dry CH2C12 (3 mL), and the mixture was stirred at room temperature for 1 h. The mixture was evaporated under reduced pressure. The crude product was passed through anion exchange resin (Amberlite IRA-400(OH), water) and cation exchange resin (Amberlite IR 120 (plus), water) to remove ions. Water was lyophilized to

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Zu molekularen Drähten 2 stapeln sich die Sn32−‐Dreiringe 1 in BaSn3. Für dessen Supraleitfähigkeit wird das ausgewogene Zusammenspiel der Wechselwirkungen zwischen lokalisierten und delokalisierten Strukturbereichen als Ursache angesehen. Der „Fingerabdruck”︁ dieses Supraleiters in der elektronischen Bandstruktur wird anhand der hier vorgestellten partiellen Elektronendichte (PED) sowie der Elektronenlokalisierungsfunktion (ELF) diskutiert.magnified image

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