Sodiated β‐Diphosphonate Carbanions: Characterization of the Tetrameric Cubane and the Hexameric Ladder Complexes [{( i PrO) 2 P(O)} 2 CHNa] 4 and [{(EtO) 2 P(O)} 2 CHNa] 6

Reaction of the methylene diphosphonates [(RO) 2 P(O)] 2 CH 2 , where R = i Pr or Et, with the metalating reagents NaH, n BuNa or the superbasic mixture t BuONa/ n BuLi leads to monodeprotonation, and formation of the complexes [{( i PrO) 2 P(O)} 2 CHNa] 4 ( 1 ) and [{(EtO) 2 P(O)} 2 CHNa] 6 ( 2 ). Single‐crystal X‐ray diffraction studies revealed that 1 is tetrameric in the solid state, with a central Na 4 O 4 cubane core. Each metal in 1 is rendered pentacoordinate by binding to three μ 3 ‐O phosphoryl bridges within the cubane, a chelating phosphoryl unit, and also a single i PrO group. In contrast, the structure of 2 is hexameric, and is composed of an unusual six‐rung Na 6 O 6 ladder core. Three distinct sodium environments as well as μ 1 ‐, μ 2 ‐ and μ 3 ‐O phosphoryl units are present within the ladder framework. No Na−C interactions are present in either 1 or 2 . All of the six‐membered NaOPCPO chelate rings display localized P=O double bonds and partial multiple bonding on the P−C−P linkages (mean P−O: 1.49 Å, and mean P−C: 1.69 Å). Ab initio molecular orbital calculations (HF/6−31G*) on model complexes indicate that the hexameric arrangement found for 2 is thermodynamically more stable than the cubane structure of 1 , in the absence of steric effects. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)