Transition metal clusters, boron atoms

The polyhedral boranes and carboranes discussed above may be regarded as boron clusters in which the single external orbital of each vertex atom helps to bind an external hydrogen or other monovalent atom or group. Post-transition main group elements are known to form clusters without external ligands bound to the vertex atoms. Such species are called bare metal clusters for convenience. Anionic bare metal clusters were first observed by Zintl and co-workers in the 1930s [2-5], The first evidence for anionic clusters of post-transition metals such as tin, lead, antimony, and bismuth was obtained by potentiometric titrations with alkali metals in liquid ammonia. Consequently, such anionic post-transition metal clusters are often called Zintl phases. 

B—C Bond Distances in Transition Metal Clusters Containing Boron Atoms"... 

There are still only a few examples of mixed metal-transition metal clusters containing boron atoms. The first to be reported was Cp2Co2Fe(CO)4B3H3 (92). This cluster results from the photolysis of 2-CpCoB4Hg [a basally substituted isolobal analog of pentaborane(9)] with Fe(CO)5. NMR spectroscopic data are consistent with the proposed structure shown in Fig. 17. 

Finally, just as BH3 coordinates to CO, so do a number of the higher borane cages. For example, B2H4(CO)2 (63), B3H7CO (64), B4H8CO (65), and BioHio(CO)2 (66) are all well characterized compounds. Because each boron atom with a coordinated CO contributes 3 electrons to the cluster count, only a few such exo-ligands can be accommodated. Dispite this, these clusters also express the close relationship between boron and transition metal clusters. 

Boron atoms in transition metal clusters Denuding the boron atom of B-H interactions in transition metal boron clusters Transition metal boride clusters at the molecular level Recent advances in the chemistry of carborane metal complexes incorporating d and /block elements The interplay of alkylidyne and carbaborane ligands at metal centres. I. Synthesis of electronically unsaturated mixed-metal complexes. II. Proton-mediated reactions... 

A simpler way to compare electron counts in boranes and transition-metal clusters is to consider the different numbers of valence orbitals available to the framework atoms. Transition metals, with nine valence orbitals (one s, three p, and five d orbitals), have five more orbitals available for bonding than boron, which has only four valence orbitals these five extra orbitals, when filled as a consequence of bonding within the framework and with surrounding ligands, give an increased electron count of 10 electrons per framework atom. Consequently, a useful rule of thumb is to increase the electron requirement of the cluster by 10 per framework atom when replacing a boron with a transition-metal atom. In the example cited previously, replacing the six borons in closo- d with six cobalts should, therefore, increase the electron count from 26 to 86 for a comparable closo cobalt cluster. Co6(CO)i6, an 86-electron cluster, meets this requirement. 

Housecroft C (1991) Boron atoms in transition metal clusters. Adv Organometal Chem 33 1-... 

These ranges show that there is variation of substituent effect with the character of the substituted a-boron atom. There are generally also noticeable jS- and y-effects on the other cluster boron atoms. For example, small )S-shift effects of either sign (depending on the cluster type, and on whether or not the substituent exhibits non-(T electronic interaction with the cluster),are usually associated with the deshielding a effects. In addition, differential /S effects of up to several ppm in the same cluster are encountered when the a atom is a transition metal center with mixed ligands that have different trans effects. y effects are often less marked ... 

---