Mixed hydrides structure

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

The relationship of Al and Ga hydrides to boranes is underlined by the formation of mixed hydrides such as HM (/a-H)2BH2 2 (M = Al, Ga) and Al (/t-H)2BH2 3. The volatile, thermally unstable GaBH has a [H2Ga(/A-H)BH2(/A-H)] infinite chain structure.21 Aluminum and Ga hydrides and their adducts are of interest as precursors to metal films and III-V semiconductors. 

In the second type of mixed hydride, hypercarbon atoms feature alongside boron atoms in the polyhedral molecular skeleton. Most of these mixed hydrides contain more boron atoms than carbon atoms, and their formulae and structures can be understood simply by isoelectronic replacement of B by C, B by C+, or BH units by C atoms in the parent borane. They are therefore known as carbaboranes, though the shorter name carboranes, coined soon after their discovery, "- is that most often used for this important category of compound and will be used here. Their now well-documented chemistry, " particularly structural and bonding aspects, is the concern of the present chapter. 

Mixed Metal Complexes. In contrast to the reaction producing [Fe5C(CO)j4 ] discussed above, treatment of Fe2(CO)g with [(jr-Cp)Mo(CO)3] at room temperature in THF yields [(7r-Cp)2Mo2Fe2(CO),Q] , for which structure (17) is suggested on the basis of i.r. data. The analogous tungsten compound has also been isolated, but no hydrides were isolated upon acidification. ... 

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