Carbonyl complexes bridging modes

Similarly, treatment of the nickel-ruthenium complex 62 with a variety of donors (CO, CNCy, NCfBu, PPh3) affords the doubly bridged complexes 78-81 (Scheme 7), wherein the donor binds to ruthenium. However, upon treatment with PhC=CH, only one carbonyl adopts a bridging mode, the second bridge being provided by a r2 vinylidene (63), derived from a 1,2-hydride shift within the PhC=CH ligand. 

When the CO pressure is increased to 10 Torr, the suhcarbonyl band increases, and a new peak at 2046 cm becomes predominant (Fig. 4b). This band is ascribed to the T2 mode of Ni(CO)4 physically adsorbed on the silica surface [26], the formation of such species being expected when Ni particles are exposed to such a high CO pressure [18,27,281. By increasing the time of contact with CO (10 Torr), the band of Ni(CO)4 at 2046 cm" progressively ws up (Fig. 4c-g), whereas that of suhcarbonyl (2080 cm" ) undergoes a gr iral depletion and almost completely disappears (Fig. 4d-g). T e complex bridg carbonyl bands are also modified as follows the stronger band at 1960 cm" increases and shifts down to 1945 cm" and the intensity of the shoulder at 1895 cm"l increases as well (Fig. 4b-g). 

While terminal and edge-bridged CO-ligands are normally found in monuc-lear and binuclear carbonyl complexes respectively, the face-bridging mode is characteristic for cluster complexes and, the bigger the nuclearity of the cluster, the more frequently jn -CO ligands are found. 

Table III shows the bonding modes that have been established for carbon monoxide in these polynuclear aggregates. In addition to the terminally bonded group 0 ) observed for the mononuclear complexes, a variety of bridging carbonyl species have also been identified, involving two-electron and four-electron donation from the carbonyl group to the cluster unit.

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