The reaction of 3,3’,4,4’-tetramethyl-2,2’-biphosphinine (tmbp) with Mn-2(CO)(10) at 140 degrees C affords [Mn-2(tmbp)(CO)(6)], a complex where tmbp acts as a 8-electron donor through its central P=C-C=P 1,4-diphosphadiene unit. An X-ray crystal structure analysis of this complex shows a delocalized 5-membered C2MnP2 ring pi-complexed to a second Mn(CO)(3) unit, which results in a disruption of the aromaticity of the two phosphinine rings. Similar behavior is observed upon reaction of 1,2-dihydro-1,2-diphosphetes with Mn-2(CO)(10). The initial product involves the incorporation of Mn(CO)(4) and Mn(CO)(3) units into the P-P bond of the ring and coordination of the C=C double bond of the diphosphete to the Mn(CO)(3) unit. Loss of CO occurs upon heating, to yield a 1,4-diphosphadiene-Mn-2(CO)(6) complex whose structure is strictly analogous to the central unit of [Mn-2(tmbp)(CO)(6)].