Mononuclear, Homo- and Heterodinuclear Nickel(I) and Palladium(I) Carbonyl Complexes

Steric bulk at the phosphine donors and the identity of the metal center significantly control the stability, structure, nuclearity, and electronic properties of PNP-stabilized monovalent group 10 carbonyl complexes. Sterically undemanding P-substituents favor dinuclear carbonyl-bridged species, while increased bulk (^<i>i</i>Pr, ^<i>t</i>Bu) stabilizes mononuclear adducts. The T-shaped nickel(I) complex [(^<i>t</i>BuPNP)Ni^I] (<b>3</b>^<b>tBu</b><b>-Ni</b>) has been synthesized, fully characterized and subsequently compared to its ^<i>i</i>Pr-substituted congener [(^<i>i</i>PrPNP)Ni^I] (<b>3</b>^<b>iPr</b><b>-Ni</b>). Both Ni(I) complexes <b>3</b>^<b>tBu</b><b>-Ni</b> and <b>3</b>^<b>iPr</b><b>-Ni</b> were subjected to 1-5 bar of CO pressure giving the mononuclear monocarbonyl complexes [(^<i>t</i>BuPNP)Ni^I(CO)] (<b>4</b>^<b>tBu</b><b>-Ni</b>) and [(^<i>i</i>PrPNP)Ni^I(CO)] (<b>4</b>^<b>iPr</b><b>-Ni</b>). From the reaction of CO (5 bar) with the previously reported dinuclear complex [(^EtPNP)Ni]₂ (<b>5</b>^<b>Et</b><b>-Ni</b>) a dinuclear compound [{(^EtPNP)Ni}₂(μ₂-CO)₂] (<b>6</b>) was obtained in which two carbonyl ligands are semibridging. Upon applying a CO atmosphere to a solution of the [(^<i>i</i>PrPNP)Pd]₂ dimer (<b>5</b>^<b>iPr</b><b>-Pd</b>), initially the mononuclear [(^<i>i</i>PrPNP)Pd^I(CO)] complex (<b>4</b>^<b>iPr</b><b>-Pd</b>) is thought to be formed, which dimerized subsequently to give [{(^<i>i</i>PrPNP)Pd^I}₂(μ₂-CO)] (<b>7</b>). The molecular structure of the dinuclear complex <b>7</b> was established by single-crystal X-ray diffraction which confirmed its dinuclear structure with a single semibridging carbonyl ligand. Finally, reaction of the heterodinuclear Ni-Pd complex [(^EtPNP)Ni-Pd(^EtPNP)] with CO gave a dinuclear Ni(0)/Pd(II)-complex <b>8</b> demonstrating favored <i>heterolytic</i> bond cleavage in the presence of π acidic ligands.