A cyclic alkyl(amino)carbene-stabilized 1,4-diborabenzene (DBB) ligand enables the isolation of 18-electron two-legged parent piano-stool Fe-0 and Ru-0 complexes, [(eta(6)-DBB)M(CO)(2)], the ruthenium complex being the first of its kind to be structurally characterized. [(eta(6)-DBB)Fe(CO)(2)] reacts with E-4 (E=P, As) to yield mixed DBB-cyclo-E-4 sandwich complexes with planar E-4(2-) ligands. Computational analyses confirm the strong electron-donating capacity of the DBB ligand and show that the E-4 ligand is bound by four equivalent Fe-P sigma bonds.