delta-Bonding in the [Pd-4(mu(4)-C9H9)(mu(4)-C8H8)](+) sandwich complex

A remarkable triple-decker sandwich complex [Pd-4(mu(4)-C9H9)(mu(4)-C8H8)][BAr4f] (BAr4f = B{3,5-(CF3)(2)(C6H3)}(4)) composed of cyclononatetraenyl anion and cyclooctatetraene as bread pieces'' and square tetrapalladium dication as meat'' (Fig. 1a) has been synthesized recently [Murahashi et al., J. Am. Chem. Soc., 2009, 131, 9888]. This complex attracted our attention because of the presence of an almost perfect square sheet composed of four palladium atoms. Such a structure could be a sign of aromatic nature of chemical bonding as it was shown to be present in the square Al-4(2-) cluster [Li et al., Science, 2001, 291, 859]. In this work we show that according to our chemical bonding analysis the bonding in the Pd-4(2+) unit of [Pd-4(mu(4)-C9H9)(mu(4)-C8H8)](+) is of delta-character among four palladium atoms, making the triple-decker sandwich complex the first synthesized compound identified as having delta-bonding in its cyclic building block when it's in solution or in a crystalline state.