Edge-bridged tetrahedral geometry

Some pentanuclear osmium cluster complexes show structures with edge-bridged tetrahedral geometries (c in Fig. 2.17). These structures have the same number of metal-metal bonds as the square-based pyramid but only triangular faces, so the stabilization of intersticial atoms is not possible there. 

In the solid state, the dianion [Rh4(CO)ii]2 possesses a distorted tetrahedral geometry with four terminal and seven edge-bridging carbonyl groups, whereas in solution it is fluxional at — 70°C (18, 74). 

On the basis of NMR spectroscopy, the structure of [Ir(Tp)H4] and [Ir(Tp )H4] has been hypothesized as supporting face-capped octahedral structure (C3J with each of the three Ir-H bonds trans to an Ir-N, and the fourth hydride capping the IrH3 face. Density functional geometry optimizations and coupled cluster calculation on [Ir(Tp)H4] indicated that a Cs edge-bridged octahedral tetrahedral structure and a Ci T]2-dihydrogen, dihydride structure are local minima.463... 

Several 60-CVE homo- and heteronuclear complexes of general formula [M4 pM v-(CO)i2]" (M, M = Fe, Ru, Co, Rh) are known. Their tetrahedral geometry is here exemplified in Fig. 9, which refers to [Co4(CO)i2] and [Co3Ru(CO)i2] , respectively. It is apparent that in both species each metal-metal edge is /z-bridged by a carbonyl group lying in the basal plane. 

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