Unsaturated Binuclear Cyclopentadienylrhenium Carbonyl Derivatives: Metal−Metal Multiple Bonds and Agostic Hydrogen Atoms
journal contributionposted on 04.08.2008, 00:00 by Bing Xu, Qian-shu Li, Yaoming Xie, R. Bruce King, Henry F. Schaefer
The cyclopentadienylrhenium carbonyls Cp2Re2(CO)n (Cp = η5-C5H5; n = 5, 4, 3, 2) have been studied by density functional theory. The global minima for the Cp2Re2(CO)n (n = 5, 4, 3, 2) derivatives are predicted to be the singly bridged structure Cp2Re2(CO)4(μ-CO) with a formal Re−Re single bond; the doubly semibridged structure Cp2Re2(CO)4 with a formal ReRe double bond; the triply bridged structure Cp2Re2(μ-CO)3 with a formal ReRe triple bond; and the doubly bridged structure Cp2Re2(μ-CO)2, respectively. The first three of these predicted structures have been realized experimentally in the stable compounds (η5-C5H5)2Re2(CO)4(μ-CO), (η5-Me5C5)2Re2(CO)4 and (η5-Me5C5)2Re2(μ-CO)3. In addition, structures of the type Cp2Re−Re(CO)n with both rings bonded only to one metal and unknown in manganese chemistry are also found for rhenium but at energies significantly above the global minima. The unsaturated Cp2Re−Re(CO)n structures (n = 4, 3, 2) have agostic Cp hydrogen atoms forming C−H−Re bridges to the unsaturated Re(CO)n group with a Re−H distance as short as 2.04 Å.