Si44- in Solution - First Solvate Crystal Structure of the Ligand-free Tetrasilicide Tetraanion in Rb1.2K2.8Si4·7NH3.

We report the first solvate structure of the silicide anion Si44-, which provides circumstantial evidence of the stability of the highly charged anion in liquid ammonia solutions. The solvate Rb1.2K2.8Si4 ·7NH3 crystallized from a mixture of the ternary compound K6Rb6Si17 with the transition metal complex [(C6H5)3P]2Ni(CO)2 [bis(triphenylphosphine)dicarbonylnickel] in the presence of the chelating agents 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) and [2.2.2]cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) in liquid ammonia. Single X-ray diffraction analysis confirms the presence of the Si44- anion in the crystal structure of Rb1.2K2.8Si4 ·7NH3, which represents the first solvate compound of the naked tetrasilicide tetraanion. All five crystallographically independent cation positions show mixed occupancy by Rb+ and K+. [ABSTRACT FROM AUTHOR]