The structural chemistry of main-group and transition-metal clusters has been set forth in the last two chapters. What more can be said about molecular clusters? Quite a bit, in fact. Although broad similarities between p-block and d-block cluster chemistries exist, we have illustrated important differences in structural preferences. The intriguing question, then, is what happens if p-block and d-block elements compete in a single cluster environment? Will the preferences of one element type dominate the other or will the merging of metal and main-group fragments generate possibilities not accessible to main-group or transition-metal systems alone. Perhaps clusters with novel hybrid properties will result.

But there is another perspective to mixed clusters. The transition-metal chemist sees the main-group fragment as a complex “ligand” through which structure and chemistry at the metal centers is perturbed. A p-block chemist may rather view metals as tools to systematically vary the structure and reactivity of a coordinated main-group moiety. Neither the cluster perspective nor the metal-complex view is wrong: one chooses a perspective optimal for the problem at hand. In this chapter we explore mixed p-block/d-block compounds as metal–ligand complexes with an emphasis on connections to organometallic chemistry. In Chapter 5 the focus will be the complementary cluster view.

Isolobal main-group and transition-metal fragments

In the first three chapters, instances were noted where the number, symmetry characteristics and occupation numbers of the frontier orbitals of a transition-metal fragment were similar to those of a main-group fragment.