Monocapped trigonal bipyramid, 906

The monocapped trigonal bipyramid is the same as a bicapped tetrahedron (23.29). 

This cluster has a monocapped trigonal bipyramidal structure in which the 18-electron rule is obeyed for only two of the six Os atoms. The situation is worse for the localized electron count and for the 18-electron rale that do not work at all in the very stable octahedral 86-electron dianion (see below). On the other hand, Wade s theory that uses the polyhedral skeleton electron pairs very well rationalizes the electron counts in large organometallic clusters. - ... 

Monocapped Trigonal Bipyramid Monocapped Trigonal Bipyromid Distorted Octahedron... 

The cluster Sng 4 has a monocapped square antiprismatic geometry. On the other hand, the cluster SM5 ]2, 68b, has a trigonal bipyramidal geometry, as shown by X-ray crystallography107. 

FBP = pentagonal bipyramid CTP = C , monocapped trigonal prism CO = monocapped octahedron DD = dodecahedron SAP = square antiprism BTP = bicapped trigonal prism. 

Rh9(CO)2iP]2+ monocapped square antiprism with interstitial P Fe3(CO)9C2BFl3, octahedral Co3(CO)9Bi, tetrahedron HRu3Fe(CO)i2N, butterfly with interstitial N or trigonal bipyramid depending on the butterfly deltahedral angle Fe3(CO)i2(CH)As, trigonal bipyramid. 

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