Structures and Bonding of η2‑Bridging CO Ligands and Their Influence on the Structures and Rearrangements of Higher Nuclearity Metal Carbonyl Cluster Complexes

Three products were obtained from the reaction of IrRu3(CO)13(μ3-H) with P­(But)3 in a hexane solution at reflux for 30 min. These have been identified as IrRu3(CO)12[P­(But)3]­(μ-H) (1; 15% yield), IrRu2(CO)9[P­(But)3]­(μ-H) (2; 29% yield), and IrRu3(CO)10(μ3-η2-CO)­[P­(But)3]2(μ-H) (3; 19% yield). Compound 1 is simply a P­(But)3 derivative of IrRu3(CO)13(μ3-H). Compound 2 is electronically unsaturated and has a vacant coordination site located on the iridium atom. Compound 3 contains a butterfly tetrahedral cluster of four metal atoms with a rare η2 triply bridging CO ligand. Compound 3 reacts with Ru­(CO)5 to yield the higher nuclearity cluster complex IrRu4(CO)12(μ4-η2-CO)­[P­(But)3]2(μ3-H) (4), which contains five metal atoms arranged in the form of an iridium-capped butterfly tetrahedron of four ruthenium atoms. Compound 4 contains a η2-quadruply bridging CO ligand. Compound 4 reacts with CO to yield the compound IrRu4(CO)14P­(But)3(μ4-η2-CO)­(μ-H), 5 which contains five metal atoms in the form of a spiked-tetrahedron. Compound 5 also contains a η2 quadruply bridging CO ligand. Compound 5 was converted to 2 in 36% yield by removal of two of the Ru groups by treatment with CO at 70 °C/10 atm. All of the new products were characterized structurally by single-crystal X-ray diffraction analyses. The η2-coordinated bridging CO ligands serve as four-electron donors. The nature of the bonding of the different types of η2-bridging CO ligands to the metal atoms in these clusters was investigated by DFT computational methods.