While isolated samples of [2-K(C7H8)]2 are resistant to further reduction by the hydride component of Li[ s Bu3BH] or K[Et3BH], the addition of K[Et3BH] to a mixture of [1-K]2/CO in benzene/THF in situ allows a more reduced product to be obtained. In this case, the reaction leads to evolution of dihydrogen (as judged by 1 H NMR spectroscopy) and the formation of the further reduced species K4[{(NON)Al}2(C4O4)(BEt3)2] ([4-K2(BEt3)]2), containing an unprecedented formally hexa-anionic [C4O4] 6-chain (Scheme 3 and Figure ). Mechanistcally, the fact that isolated samples of [2-K(C7H8)]2 are not reduced (either chemically or electrochemically) suggests that the formation of ([4-K2(BEt3)]2 involves a reduction event which occurs prior to the dimerization of the carbene intermediate I3.