Notes Bull. Korean Chem. Soc. 2010, Vol. 31, No. 12 3815 DOI 10.5012/bkcs.2010.31.12.3815 Effects for Charge Transfer in Pd(II) Complexes along Various Terminal Ligands Mahreen Arooj, Kyungok Park, † and Jong Keun Park * Department of Chemistry Education, Research Institute of Natural Science and Educational Research Institute, Gyeongsang National University, Jinju 660-701, Korea. * E-mail: mc7@gnu.ac.kr † Samsung Advanced Technology Training Institute, HRD Center, Samsung Electronics, Korea Received July 2, 2010, Accepted October 6, 2010 Key Words: Charge transfer, Heterocyclic compound, Pd(II) complex, DFT X X Pd PH 2 PH 2 Pd EDG(δ + ) H 2 P H 2 P n n δ δ + EWG(δ ) (δ + )EDG (δ )EWG (δ + )EDG (δ )EWG EDG(δ + ) EWG(δ ) δ + δ n = 1-3; X = S, Se EDG: electron donating group EWG: electron withdrawing group L L' L L' A Type 1: NH3 NH3 C Type 6: NH3 PH3 2: CH3O ‒ CH3O ‒ 7: PH3 Cl ‒ 8: CH3 ‒ PH3 B Type 3: PH3 PH3 9: Cl ‒ F ‒ 4: Cl ‒ Cl ‒ 10: CH3 ‒ F ‒ 5: F ‒ F ‒ 11: CH3O ‒ F ‒ Scheme 1. Schematic diagram of the charge transfer in Pd complex along with terminal and bridging ligands Heterocyclic compounds such as thiophene and selenophene with their plane π-conjugated structures are of immense interest as they give rise to wide range of applications. 1 The charge vari- ations of X (X = S, Se) on the bridging ligand in Pd(II) metal complexes were influenced by the electron donating and/or withdrawing groups at the terminal ligands. Many researchers have used variety of electron-rich or electron-deficient substi- tuent which induces intra-molecular interactions thus controlling the geometry of the structures. 2-3 Therefore, rendezvous of suit- able substituent is an important factor that can influence the intra-molecular geometry of the complexes via atomic charge variations between various substituents. Phetmung et al. indi- cated the significant character of the S···S inter- and intra-mole- cular interactions in controlling the reactivity of sulfur com- pounds, the actual conformation, and crystal packing. 4 In addi- tion, Hayashi et al. reported the Se···Se contacts in structural and theoretical investigations which stabilized the structures of selenoxides. 5 In an experimental study, Seppälä group showed the attractive Se···Se interactions between uncharged molecules which lead to the formation of dimmer in supra-molecular struc- tures. 6 The chalcogen have a strong inclination to act as bridging ligands and a number of bimetallic group 10 complexes with these bridging chalcogen have been characterized. 7 The main goal of this research work is to theoretically investigate the vari- ation of the atomic charges of X on bridging heterocyclic ligands along the electron donating and/or withdrawing groups at the terminal ligands. Whether the electron donating and/or with- drawing groups at the terminal ligand instigate modification of the atomic charge of the X on bridging ligands? Are there any relationship between the electron-property of the terminal ligand and the (X … X) interaction of inta-bridging ligand? What are the effects of changing heteroatoms from sulfur to selenium? The research work reported here intends to counter all the above- mentioned queries. Pd(II) complexes having oligothiophene were optimized with 6-31g* for C, P, H, O, Cl, F, and N atoms and LANL2DZ for Pd atom at the density functional theory (B3LYP) level using Gaussian 03 package. 8,9 While, Pd(II) complexes having selen- ophene as bridging ligand were optimized with 3-21g* basis sets at the B3LYP level. Natural Bond Orbital (NBO) analysis was performed in order to investigate the charge distribution in all complexes. To confirm the existence of stable structures, the harmonic vibrational frequencies of the structures were anal- yzed at the B3LYP level. The schematic presentation of the atomic charge transfer in Pd complex in conjunction with the terminal and bridging ligands is depicted in the Scheme 1. The total number of combinations of ligands which are applied as terminal ligand are 11. Pd(II) complexes can be classified into three types, which are depended upon the kind of terminal ligands as following: A (1~2) complexes with four electron- donating groups at the terminal ligands, B (3~5) complexes with four electron-withdrawing groups, C (6~11) complexes with two electron-donating and the other two electron-with- drawing groups. Pd(II) complexes having phosphine-substituted monothiophene, bithiophene, and terthiophene bridging ligands are designated as a , b, and c, respectively. Here, we have attempted to find out the influence of terminal ligands which is most suitable in instigating charge transfer and X···X distance. As in all Pd(II) complexes, the variation in terminal ligand environment has shown profound effect on structures of Pd(II) complexes. Therefore, an investigating piece of property data can be found by changing the atomic charge of X in Pd(II) complexes with different terminal ligands. Due to the substitution of terminal ligands of the A, B, and C types, the variations of the atomic charge of X and X···X distance has been observed. In optimized Pd(II) complexes, each metal cen- ter adopts a square planer geometry in which two positions are