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Herein, we report the first 1,4-diphosphinine-1,4-diide compound [(ADC Ph )P] 2 ( 5-Ph ) (ADC Ph = PhC{(NDipp)C} 2 ; Dipp = 2,6-iPr 2 C 6 H 3 ) derived from an anionic dicarbene (ADC Ph ) as a red crystalline solid. Compound 5-Ph containing a 16pi-electron planar fused-tricyclic ring system was obtained by the 4e reduction of [(ADC Ph )PCl 2 ] 2 ( 4-Ph ) with Mg (or KC 8 ) in a quantitative yield. Experimental and computational results imply that the central 8pi-electrons C 4 P 2 ring of 5-Ph , which is fused between two 6pi-electrons C 3 N 2 aromatic rings, is antiaromatic. Thus, each of the phosphorus atoms of 5-Ph has two electron-lone-pairs, one in a p-type orbital is in conjugation with the C=C bonds of the C 4 P 2 ring, while the second resides in a sigma-symmetric orbital. This can be shown with the gold complex [(ADC Ph )P(AuCl) 2 ] 2 ( 6-Ph ) obtained by reacting 5-Ph with (Me 2 S)AuCl. A mixture of 5-Ph and 4-Ph undergoes comproportionation in the presence of MgCl 2 to form the intermediate oxidation state compound [(ADC Ar )P] 2 (MgCl 4 ) ( 7-Ph ), which is an aromatic species. © 2020 Wiley-VCH GmbH.