New symmetrical [p-CN-C6H4C(O)CH2PPh2(CH2)2PPh2CH2C(O)C6H4-p-CN]Br2 (1) and unsymmetrical [PPh2(CH2)2PPh2CH2C(O)C6H4-p-CN]Br (2) phosphonium salts were prepared by reacting Bis(diphenylphosphino)ethane (dppe) with p-CN-C6H4C(O)CH2Br in acetone, in 1:2 and 1:1 ratios, respectively. Treatment of these salts with an appropriate alkali led to symmetrical p-CN-C6H4C(O)C(H) PPh2(CH2)2PPh2C(H)C(O)C6H4-p-CN (3) and unsymmetrical PPh2(CH2)2PPh2C(H)C(O)C6H4-p-CN (4) ylides.The reaction of 3 and 4 with [MCl2(cod)] (M ¼ Pd or Pt; cod ¼ 1,5-cyclooctadiene) in 1:1 ratio, using dichloromethane as a solvent, led to mononuclear seven membered metallocyclic ring complexes ([MCl2(p-CN-C6H4C(O)C(H)PPh2(CH2)2PPh2C(H)C(O)C6H4-p-CN)] (M ¼ Pd (5) and Pt (6), having C,C coordination mode) and mononuclear six membered ring complexes [MCl2(PPh2(CH2)2PPh2C(H)C(O)C6H4- p-CN)] (M ¼ Pd (7) and Pt (8), having P,C coordination mode), respectively. Characterization of the resulting compounds were performed by elemental analysis, IR, 1H,13C and 31P NMR spectroscopic methods. As well, the unequivocal structure of compound 5 (seven membered pallada-cycle complex) was characterized crystallographically. In the symmetrical complexes, with seven membered metallocyclic rings, the title ylide is coordinated to the metal through two ylidic carbon atoms. Using theoretical methods, Density-function theory (DFT) calculations at the BP86/def2-SVP level of theory, the structures of the unsymmetrical six membered pallada- and platinacyclic complexes were compared with the structures of the symmetrical seven membered pallada- and platinacyclic complexes. The strength and nature of donoracceptor bonds between the phosphorus ylide (Y ligand) and MCl2 fragments in the [YMCl2] (M ¼ Pd, Pt, Y ¼ p-CN-C6H4C(O)C(H)PPh2(CH2)2PPh2C(H)C(O)C6H4-pCN,PPh2(CH2)nPPh2C(H)C(O)C6H4-p-CN, n ¼ 1e2, were carried out by Natural bond orbital analysis (NBO) and energy-decomposition analysis (EDA) as well as their natural orbitals for chem
