M4 tetrahedra

The structures are shown in Fig. 26.8c and d and differ in that, whereas the Ir compound consists of a tetrahedron of metal atoms held together solely by M-M bonds, the Rh and Co compounds each incorporate 3 bridging carbonyls. A similar difference was noted in the case of the trinuclear carbonyls of Fe, Ru and Os (p. 1104) and can be explained in a similar way. The M4 tetrahedra of Co and Rh are small enough to be accommodated in an icosahedral array of CO ligands whereas the larger Ir4 tetrahedron forces the adoption of the less dense cube octahedral array of ligands. 

Two very different clusters comprised of inner and outer M4 tetrahedra are known. 

Structures involving M4 tetrahedra sharing vertices, edges or faces... 

Reentrant superconductivity was first discovered with ErRh4B4. [204] The chain of trans edge condensed M4 tetrahedra in which the coordinating atoms lie above all faces of the tetrahedra has been called a Tfetraederstern by Schubert. This very common feature links the structural chemistry of condensed cluster compounds with that of the intermetallic phases. [205]... 

LaNij has the CaCu, structure, space group P6lmmm [26] the hexagonal metal lattice is shown in Fig. 4. The crystal structure of LaNi Dy has been determined [27, 28] and is illustrated in Fig. 5. There are three types of interstitial D sites LUyNi tetrahedra, and M4 tetrahedra. The unit cell is doubled along the c-axis because of the formation of a superlattice which is a consequence of long-range correlations between occupied and unoccupied... 

Both the complexes 8.7 and 8.7a contain triangular M3(CO)9 systems and it appears that this metal cluster has some special stability. It is indeed becoming clear that metal clusters of M4 tetrahedra and M[Pg.294]

Figure 5 A 3D projection of the structure of an amphibole (K,Na)(Ca,Na)2(Fe, Mg,Fe , Al)5 (Si,Al)s022(0H,F)2. Double chains of Si04 tetrahedra are situated face-to-face and back-to-back the large cations (the A and M4 sites larger spheres) are situated between back-to-back chains, whereas the small cations (the Ml, M2, and M3 sites smaller spheres) are situated between face-to-face chains. The ordinary oxygen sites are not plotted because they are already represented by the corners of the tetrahedra however, the (OH, F) site (large light-ornamented sphere for the O along with a black dot for the H) is an additional site at the center of pseudohexagonal holes in the array of apical oxygens, (After Ref. 4.)...

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