Six-coordinate interstitial sites

Because the radius of an octahedral cavity is 0.414 times the metallic radius of the metal atoms on the surface of the cluster, only the first-row main-group atoms can be incorporated in this site. There are several octahedral carbido- and nitrido-clusters, but examples are also known that incorporate hydrogen and boron. A series of related clusters include [Ru6H(CO)i8] , [Ru6B(CO)i7] , [RueQCO)] ], and [Ru6N(CO)i6] , all of whieh contain a fully encapsulated interstitial heteroatom lodged in the center of the octahedral cavity. 

The first example of a six-coordinate interstitial hydride was [Ru6H(CO)i8] the unexpected location was originally deduced indirectly from the close-packed arrangement of the carbonyl ligands which left no space on the cluster surface for an H atom. This structural assignment has since been vindicated by a neutron diffraction study. Similarly, neutron diffraction studies of the hexacobalt cluster 

The hexacobalt and hexaosmium cluster anions, [Co6P(CO)i6] and [Os6P(CO)i8] are interesting examples which illustrate the effects of introducing a larger, second-row, main-group element into a six-coordinate interstitial cavity. They also enable a comparison of the differences between the cavity sizes for first and third-row transition metals to be made. 

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