Cluster Expansion Reactions of Group 6 Metallaboranes. Syntheses, Crystal Structures, and Spectroscopic Characterizations of (Cp*Cr)2B5H9, (Cp*Cr)2B4H8Fe(CO)3, (Cp*Cr)2B4H7Co(CO)3, and (Cp*Mo)2B5H9Fe(CO)3

The targeted high-yield syntheses of a number of new clusters by the controlled addition of BH, Fe(CO)3, or Co(CO)3 fragments to dinuclear group 6 metallaboranes are reported. The reaction of the chromaborane, (Cp*Cr)2B4H8 (1), Cp* = η5-C5Me5, with BHCl2·SMe2 results in a cluster expansion reaction giving the green-brown diamagnetic species (Cp*Cr)2B5H9 (2) in 43% yield. The six skeletal electron pair (sep) cluster has a structure based on a trigonal bipyramidal Cr2B3 unit with adjacent Cr2B faces capped by BH3 fragments. Reaction of 1 with the dinuclear metal carbonyls Fe2(CO)9 and Co2(CO)8 leads to the isolation of the mixed-metal metallaboranes (Cp*Cr)2B4H8Fe(CO)3 (3) and (Cp*Cr)2B4H7Co(CO)3 (4) in yields of 76 and 87%, respectively. 3 and 4 can also be viewed as six-sep bicapped trigonal bipyramidal clusters with the metal carbonyl occupying one of the capping positions. Alternatively, they can be viewed as complexes between M(CO)3 transition metal fragments (M = Fe, Co) and the four-electron (Cp*Cr)2...