Trigonal pyramidal structures, atomic clusters

Cobalt exhibits a characteristic tendency to form clusters of the formula RCCo3(CO)9, which have trigonal-pyramidal structures. The cobalt atoms occupy the three positions at the base of the pyramid, and the carbon group lies at its top vertex. These cobalt-carbon clusters are prepared by reactions of RCX3 with Co2(CO)8 or NaCo(CO)4 in the presence of Lewis bases ... 

As for tetranuclear clusters, pentanuclear transition metal clusters are only capable of partially encapsulating a main-group heteroatom. The c/o5o-pentanuclear cluster, the trigonal bipyramid, has an interstitial cavity of similar dimension to that of the tetrahedron, and must therefore open-up if it is to accommodate a semi-interstitial atom. The most commonly observed M5E clusters (E = main-group atom) adopt square based pyramidal structures, which can be derived from the trigonal bipyramid by M-M bond cleavage or can, alternatively, be viewed as /i/Jo-octahedral species (Scheme 2). 

In the above compounds, the structural motif is the M4S4 cubane, but these closed structures will not incorporate three //2-sulfide bridges easily. A different self-assembly route leads to MoFe clusters that have a different composition but do contain a sulfur-voided cubane held by three //2-sulfides. The clusters MFe4S6Ls are shown as (15). Their Fe4(//3-S)3(//2-S)3 portions structurally resemble the part of the FeMoco farthest from molybdenum. These pentametallic clusters have been used to calibrate Fe EXAFS of the FeMoco. Another important aspect of these clusters is that three iron atoms have trigonal pyramidal geometry like the belt iron atoms of the FeMoco, with three sulfides in the basal positions and a phosphine in the axial position. A series of basket ... 

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