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 [(Tp(Np)CO)(2)(mu-N-2)] and Tp(Np)Co(CO) show ''bent'' molecules, in which the fourth ligand (N-2, CO) is bent away from the pseudo-threefold axis of the TpCo-moiety by 27-38 degrees. Magnesium reduction of Tp(t-Bu,Me)Co(CO) yielded [Tp(t-Bu,Me)Co(mu-CO)]Mg-2(THF)(4) which was also structurally characterized; the reduced carbonyl is ''linear''. Extended Huckel 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 (Co-1, d(8)), COLi (Co-0, d(9)), and I (Co-II, d(7)). The actual and theoretical structure determinations were in good agreement. Based on these results we suggest that d(8) TpCo-L complexes and, by analogy, isoelectronic CpM-L complexes assume ''bent'' structures.