Hydride ligand face-bridging

A series of recent reports have shown that reacting nickelocene with various hydrocarbons in a reducing environment results in the formation of a family of novel complexes, as shown in Scheme 37. For instance, the reaction of nickelocene with 1-pentene or 1-hexene in the presence of metallic sodium gave the red, paramagnetic alkylidyne-hydrido clusters (NiCp)3(/W3-CR)(/U3-H), 133 (R = Bu, (CH2)4Me) in addition to the known (NiCp)3(/X3-GR), 134, and (NiCp)4(ju-H) (135, Scheme 37). The solid-state structure of 133 shows that the bridging hydride ligand caps one face of the Ni3 moiety. The solid-state structure of 135 has also been described. Closely related trinickel /X3-alkylidynes have been obtained from very different routes, by reaction of Ni(acac)Gp or NiCp2 with LiMe vide infra). 

The hydride ligands are believed to occupy -bridging positions over the large open faces—they are shown as dashed circles. 

Pt(l) via an ailylic link and to Pt(2) via an eneyl link. Potential energy minimization calculations (159) indicate that the chemically distinct hydride ligands [which were originally assigned terminal sites on Os(l) and Os(2) (55)] bridge the Os(l)-Os(2) edge and the Os(l)-Os(2)-Os(6) face. 

Fig. 56. The molecular structure of HFeCo3(CO)9(P(OMe)3)3, with methoxy groups removed for clarity (Ref. 238). The hydride ligand is clearly shown symmetrically bridging the Co3 face, and is displaced 0.978 (3) A from it...

In the majority of its metal complexes, the hydride ligand normally functions in the /x1 (terminal), /tx2(edge-bridging), and /x3 (triangular face-capping) modes. Research efforts in recent years have led to the syntheses of an increasing number of high-coordinate (/u4, /u5, and /x6) hydride complexes. 

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