Structural Distortion of the TpCo-L Fragment (Tp = tris(pyrazolyl)borate). Analysis by X-ray Diffraction and Density Functional Theory

The crystal structures of coordinatively unsaturated [(TpNpCo)2(μ-N2)] and TpNpCo(CO) show “bent” molecules, in which the fourth ligand (N2, CO) is bent away from the pseudo-threefold axis of the TpCo-moiety by 27−38°. Magnesium reduction of Tp t -Bu,MeCo(CO) yielded [Tp t -Bu,MeCo(μ-CO)]2Mg(THF)4 which was also structurally characterized; the reduced carbonyl is “linear”. Extended Hückel theory (EHT) and density functional theory (DFT) have been used to analyze the electronic structures and structural preferences of the TpCo-L fragment with L = CO (CoI, d8), COLi (Co0, d9), and I (CoII, d7). The actual and theoretical structure determinations were in good agreement. Based on these results we suggest that d8 TpCo-L complexes and, by analogy, isoelectronic CpM-L complexes assume “bent” structures.