Edge-Bridged Tetrahedral Molecules

Compounds having the bridged tetrahedal structural type are listed in Table XVII. Also listed in the table are EM4 molecules that have a similar edge-bridged tetrahedral core. The examples known are tetrametal systems with a PR group (50). Like the E2M3 described previously, compounds 50 have eight skeletal electron pairs. Their electron distributions obey the 18-electron rule at the metals however, to do this the phosphorus atoms become 10-electron centers and are formally hypervalent. 

A more common arrangement for EM4 species is the square pyramidal array with E occupying the apical vertex (51), which is related to the octahedral E2M4 (52) and EM5 (53) arrangements. Note that 51 is the structural isomer of 46 where the E atom adopts the apical rather than the basal position. These compounds are listed in Tables XVIII-XX, whereas Table XXI lists a number of more complicated molecules that have several 

Theoretical analyses of the bonding in M4E2 clusters have been made.85311312 The structures show formation of a weak bonding E-E interaction that could explain the high electron count, but it was found that the HOMO is not E-E bonding. Rather, it is M-M antibonding, and 

Closely related are [Bi4Co9(CO)16]2 and [Bi8Co14(CO)2o]2-, the metal cores of which are given as 62A/62B and 63. They may be viewed as tetracapped, tetragonally distorted M8 arrays that also possess interstitial metal atoms. Compound 63 consists of two of the 62 metal frameworks fused about one of the Co4 faces. Compared to 59, 62 is seen to be missing two of the ju4-E groups. In addition, the Co-Co bonds parallel to the C4 axis of the molecule have been broken and have been replaced with four Bi-Coimerstitial interactions. The result is that 61 can be viewed as a cuboc-tahedron, which is more apparent if the compound is viewed from a different perspective (62B). This also emphasizes the close-packed nature of the metal array. 

In addition to bridging square arrays of metal, E fragments may also adopt a /As-configuration. Complexes of this type have been summarized  

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