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. For the first time, we discovered delocalized δ-bonding in a condense-phase [Pd 4 (μ 4 -C 9 H 9 )(μ 4 -C 8 H 8 )] + triple-decker sandwich complex using the Adaptive Natural Density Partitioning method.