Diphosphacyclopentadienide anion

Tri- tert-butyl-1,3,5 -triphosphabenzene (128, Scheme 53) undergoes a ring contraction over a potassium mirror,159 with the elimination of potassium phosphine and formation of the aromatic 2,4,6-tri-tot-butyl-1,3-diphosphacyclopentadienide anion 129 with three bulky substituents (Scheme 53). The driving force of this reaction could be the higher aromaticity of the five-membered ring. 

The potassium and lithium salts of the 2,4,5-tri-/-butyl-l,3-diphosphacyclopentadienide anion (a 1,3-diphospholide anion) 28 were examined crystallographically <20000M219>. Short bond lengths in the ring were taken to indicate extensive electron delocalization. 

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