δ-Bonding in the [Pd4(μ4-C9H9)(μ4-C8H8)]+ sandwich complex

A remarkable triple-decker sandwich complex [Pd(4)(μ(4)-C(9)H(9))(μ(4)-C(8)H(8))][BAr(f)(4)] (BAr(f)(4) = B{3,5-(CF(3))(2)(C(6)H(3))}(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)(μ(4)-C(9)H(9))(μ(4)-C(8)H(8))](+) is of δ-character among four palladium atoms, making the triple-decker sandwich complex the first synthesized compound identified as having δ-bonding in its cyclic building block when it's in solution or in a crystalline state.