Induced by Particular Ligands

Some post-transition elements (or the corresponding radicals) containing 3 or more electrons in their valence shell are able to assist the formation of clusters by bonding to several metal atoms. Typical examples of this behaviour are the extraordinarily easy syntheses of large series of compounds such as Co3 (CO)9 Oi3-E) (E = Al, CR, CX, GeR, P, As, PS, S, Se, PR, SR) 207 209) and Fe3 (CO)9 (ju3 -E)2 (E = S, Se, Te, NR, PR). This type of stabilization is usually found in trinuclear clusters although a few examples in tetranuclear clusters are known, for instance  

As shown in Fig. 1, the two hetero-atoms are above and below the centre of a rectangle of cobalt atoms. As a first approximation the coordination around the heteroatoms corresponds to a pyramid in which the axial position is occupied by a lone-pair of electrons (or by the Ph group) while the four equatorial positions formally correspond to the four electrons donated by the hetero-atom. 

A much more subtle case of stabilisation due to the presence of face-bridging ligands is found in the two tetrahedral clusters Co4Cp4(/i3-H)4 (dec. ca. 300 °C)134) and Ni4Cp4( 3-H)3 (dec. ca. 320 °C)133). The structure of the last compound is shown in Fig. 2. 

Stabilization due to relief of steric crowding is probably involved in the following remarkable reactions which take place at atmospheric pressure  

Number of M-M bonds Idealized symmetry M-M in mixed clusters 

---