Five-membered cyclopalladated rings: Cambridge structural database analysis of geometrical parameters and 'aromatic' character

The relationship between the planarity of five-membered cyclopalladated rings and calculated aromaticity indices has been investigated. Geometrical parameters for 126 crystal structures containing the fragments [Pd(NXCY)A2] (X, Y = C or N; A = any ligand) were obtained from the Cambridge Structural Database. These were classified in terms of planarity by measuring the displacement of the N atom bonded to the Pd from the calculated mean plane formed by the other four atoms. The same classification was performed using the displacement of the Pd atom and the results compared. 'Aromaticity' indices for the five-membered cyclopalladated rings, V and HOMA, were also calculated based on experimental bond lengths and typically used in the analysis of five- or six-membered organic heterocycles. The correlation between calculated 'aromaticity' indices and planarity of the rings has been shown to be good. The stability and/or reactivity of the five-membered cyclopalladated rings suggested by the sequence of 'aromaticity' indices agrees well with the reactivity series of these compounds previously found for ligand-exchange reactions.