CRYSTAL-STRUCTURE OF A CLOSO 6-VERTEX METALLOBORON CLUSTER, 1,2-(ETA-5-C5H5)2CO2B4H6, WITH HYDROGEN-ATOMS BRIDGING CO2B TRIANGULAR FACES

A single-crystal X-ray diffraction investigation of the title compound established the structure as an octahedral Co2B4 cage with the metal atoms occupying adjacent vertices and each cobalt coordinated to an rj5-cyclopentadienyl ring, in agreement with the geometry originally proposed from nB and 'H NMR data. Two crystallographically equivalent hydrogen atoms occupy face-bridging positions approximately over the centers of the two Co2B triangular faces, in a manner very similar to face bridging of trimetallic faces in metal-cluster compounds. This structure represents the first instance in which face-bridging hydrogen atoms on an M2N face (where M and N are metal and nonmetal atoms, respectively) have been located and refined and is the second example of a face-bridging hydrogen on a closo polyhedral boron cage (after CB5H7) to be precisely located. The Co-Co bond distance is 2.557 (1) A, the longest such distance thus far known in a cobalt-boron cage compound; the unusual length is attributed to the presence of bridging hydrogen atoms on both sides of the Co-Co vector, which are thought to produce a lowering of the Co-Co bond order. The implications of these findings with respect to other closo metalloboranes and metallocarboranes containing “extra” hydrogen atoms are discussed. Crystal data: Mt = 297.4; space group P2x/m a = 7.601 (5), b = 9.060 (4), c = 9.793 (5) A; /3 = 108.07 (5)°; V= 641 (1) A3; n(Mo Ka) = 26.5 cm-1; p(calcd) = 1.544 g cm-3 for Z = 2. The structure was refined by full-matrix least-squares methods to a final R value of 0.039 for the 994 reflections for which F01 > 3 <r(F02). © 1979, American Chemical Society. All rights reserved.