Face-bridging hydrogen

Whereas edge- and face-bridging hydrogen atoms are associated mainly with metal-metal orbitals already present in the tetrahedral skeleton (see last section) and therefore have limited steric requirements, terminal hydrides occupy a full coordination position. It is not surprising then that the only examples of tetrahedral species with terminal hydrides contain 11 and 10 carbonyls, [Ir4(CO)nH] and [Ir4(CO)ioH2]2 . (The assignment for [Ir4(CO)nH] is based on the structure determination of the analogous [Ir4(CO)nBr] and on the similarity of the ir spectra of the two species (11).)... 

Fig. 54. The structure of H4Co4Cp4, indicating the face-bridging hydrogen positions as determined from an X-ray study (Ref. 230)...

Analogously, the bonding of a jU3-hydrogen to a triangular face of the cluster corresponds to the donation of an electron pair from a face-localized orbital to the l5 orbital of the proton. Clusters with face-bridging hydrogen ligands will have the same number of valence electrons than the parent anion. 

If this is so, then in higher clusters, where more than one face is available, a more extensive olefin chemistry may be anticipated (see below). Certainly, in complexes of this nature, the presence of bridging hydrogen atoms has been found to impose severe restrictions on the reactivity of molecules, and this may, in part, be associated with these steric requirements. 

BflHjo. and BioHi4 in Fig. 2), i.e., these BHB bridging hydrogen atoms are located roughly over the centers of incomplete faces of the fundamental polyhedron. 

Although not realized until recently, in all probability a bridge hydrogen, if forced to assume a position upon the cage [as is the case with CB5H7 (III-C4)], would locate over a triangular face (3) and would hence be associated with three borons via a 4-center bond as illustrated in IX-N5. 

Onak s CB5H7 (III-C4) 101) is under investigation by Beaudet 3), who has determined that the lone 666-bridge hydrogen is almost certainly above a triangular face. This undesirable situation (a violation of rule 2p) probably accounts for its instability, i.e., in the presence of a Lewis base (proton acceptor) the related closo-anion CBsHe" is produced 116). More recently, R. R. Rietz has reported (private communication, 1974) a second closo-carborane, CBgHg. 

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