Cyclopentadienyl Ruthenium, Rhodium, and Iridium Vertices in Metallaboranes: Geometry and Chemical Bonding

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Abstract
Most cyclopentadienylmetallaboranes containing the vertex units CpM (M = Co, Rh, Ir; Cp = η5-cyclopentadienyl ring, mainly η5-Me5C5) and CpRu donating two and one skeletal electrons, respectively, have structures closely related to binary boranes or borane anions. Smaller clusters of this type, such as metallaborane analogues of arachno-B4H10 (e.g., (CpIr)2B2H8), nido-B5H9 (e.g., (CpRh)2B3H7 and (CpRu)2B3H9), arachno-B5H11 (e.g., CpIrB4H10), B6H62- (e.g., (CpCo)4B2H4), nido-B6H10 (e.g., CpIrB5H9 and (CpRu)2B4H10), and arachno-B6H12 (e.g., (CpIr)2B4H10), have the same skeletal electron counts as those of the corresponding boranes. However, such clusters with eight or more vertices, such as metallaborane analogues of B8H82- (e.g., (CpCo)4B4H4), arachno-B8H14 (e.g., (CpRu)2B6H12), and nido-B10H14 (e.g., (CpRu)2B8H12), have two skeletal electrons less than those of the corresponding metal-free boranes, analogous to the skeletal electron counts of isocloso boranes relative to those of metal-free deltahedral boranes. Some metallaboranes have structures not analogous to metal-free boranes but instead analogous to metal carbonyl clusters such as 3-capped square pyramidal (CpRu)2B4H8 and (CpRu)3B3H8 analogous to H2Os6(CO)16 and capped octahedral (CpRh)3B4H4 analogous to Os7(CO)21. In the metallaborane structures closely related to metal-free boranes, the favored degrees of BH and CpM vertices appear to be 5 and 6, respectively.