Bicapped square antiprismatic complexes

T. Baker, DuPont Central Research In reference to your so-called iso-closo iridaborane complex, H(PPh3)2IrB9H9, we have recently published two papers (1,2) dealing with isoelec-tronic, isostructural ten-vertex ruthenacarborane complexes and have demonstrated that these structures are related to the common closo bicapped square antiprismatic structure by the removal of two electrons (i.e. 2N skeletal electrons for an N vertex polyhedron). Such complexes have been referred to as hyper-closo to imply that the electronic unsaturation is not primarily metal-based (as in, for example, nido-(PPh3)2RhC2B8-Hx 2 (3) or closo-(PPh3)ClRh(1,7-C,BQH,) (4), but is delocal-... 

The only complex with a non coordinating anion to be characterised fully is [Ce(phen)4(MeCN)2] (C104)3-3MeCN, which has 10-coordinate cerium in a bicapped square-antiprismatic geometry. 

Coordination number 10 has been demonstrated in [La(EDTA) (H20)4] and presumably occurs in various complexes of Th and U, such as the tropolone complexes [M(02C7H5)5]". A bicapped square antiprismatic arrangement has been found in K4Th(02CC02)4 4H20.17... 

Alternatively, a more spherical cluster, such as a bicapped square antiprism would incorporate a C2 moiety with a C-C separation of 1.47 A and the M-M and M-C separations between 2.54 and 2.80, and ca. 1.8-2.2 A, respectively. The C2 unit is not stabilized in a square antiprismatic cavity or in other frameworks, because the inter-centroid distances are > 2.0 A, leading to C-C bond cleavage and formation of dicarbido complexes. The strength of the metal-carbide bonding is increased at the expense of the C-C bond. However, the cavity in [Nii6(C2)2(CO)23]4, is large enough to accommodate two C2 moieties. 

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