The crystallographic and quantum mechanical analysis of some palladium(II) n?heterocyclic carbene complexes

In this thesis, molecular and crystal structures of trans?[1,3?Bis(2,4?dimethylphenyl)? imidazolidin-2?ylidene]dichloro(triphenylphosphine?kappa P)palladium(II), trans?Bis[1,3? bis(2,4?dimethylphenyl?imidazolidin?2?ylidene)]dichloropalladium(II) and trans?Bis[1,3? bis(methoxyethyl)? 4,5?bis(2,4,6-trimethylphenyl)?imidazolidin?2?ylidene)]dichloropalladium(II) were determined by single crystal X?ray diffraction technique. Bonding situations in the complexes are investigated by means of Natural Bond Orbital and Fragment Analysis based on Density Functional Theory in the framework of quantum mechanical principles. It is proved both by theoretically and experimentally that the leading contribution to the stability of complexes is achieved by the donation of electrons from nitrogen atoms to the unoccupied p(pi) orbital of carbene carbon atom in N?Heterocyclic Carbene (NHC) ligands. It is stated that NHC ligands in the complexes under discussion are contributed not only to sigma? but also to the pi?donation and pi?back donation processes. The presence of bulky substituents on the carbon atoms at 4 and 5 positions of the imidazolidin ring has adverse effect on the sigma?donation ability of NHC ligands. Besides the existence of two trans?positioned NHC ligands bonded to Pd(II) metal makes augmentative contribution to the sigma?donation of the complexes.