Carbonyl Clusters Having Interstitial Atoms

Metal Carbonyl Clusters Having Interstitial Atoms 

Many metal carbonyl clusters have interstitial atoms or groups located in the eenter of the polyhedron. Such interstitial atoms may be a light atom sueh as boron, carbon, or nitrogen a post-transition element such as germanium, tin, or antimony or a transition metal. Interstitial atoms most frequently provide all of their valence electrons as skeletal electrons since all of their valence orbitals are neeessarily internal orbitals because of the location of the interstitial atom in the center of the polyhedron. Exceptions to this rule may occur when some of the valence electrons of the interstitial atom occupy orbitals of symmetries which cannot mix with any of the molecular orbitals arising from the polyhedral skeletal bonding. 

This process corresponds to the diamond-square portion of the diamond-square-diamond process involved in polyhedral rearrangements. For example, rupture of six edges in this manner from an icosahedron can give a euboetahedron as follows  

An example of electron counting in a metal carbonyl cluster with an interstitial atom is illustrated by Ruf,C(CO)i7, which has the 14 skeletal electrons required for a globally delocalized octahedron by the following electron-counting scheme  

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Metal Carbonyl Clusters Having Interstitial Atoms... 

The first syntheses of carbonyl clusters containing an interstitial nitrogen atom involved the use of NO" " [Eqs. (45) and (46)] (3). In the case of rhodium, the major product was suggested to be [Rli6(CO)i4(NO)], but no further data have been reported on this. The reaction of NO with the mixture of iron carbonyls, Fe(CO)s and [Fe2(CO)8] , yielded two clusters... 

Many main-group atoms have been found to occupy interstitial positions in transition metal carbonyl clusters and there are now examples of H, B, C, N, O, Si, P, S, Ge, As, Sn, and Sb atoms being encapsulated, either fully or partially, within a metal skeleton. In general, the size limitations associated with the internal cavity of the cluster determine the type of atom which can occupy the interstitial site. Although the radius of the cavity is determined primarily by the nuclearity and geometry of the cluster, the size of the metal atoms relative to that of the interstitial atom is a critical factor, and clusters with larger cavities are observed as the ratio of the covalent radius of the main-group atom to that of the transition metal atoms increases. 

Os, and having the same essentially octahedral skeletons, nevertheless differ appreciably in the disposition of the attached carbonyl groups. The incorporation of interstitial (encapsulated) atoms such as C, H, S, N, P and, more recently, is a widespread and frequently stabilizing feature of these clusters. Carbido clusters are the most common the C contributing 4 electron.s... 

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