Ligands: bonding nature

This work has allowed us to see the evolution of the rare-earth — ligand bonding nature inside the LnM04 series. 

Vibrational spectra of transition metal complexes and the nature of the metal-ligand bond. D. W. James and M. J. Nolan, Prog. Inorg. Chem., 1968,9,195-275 (198). 

The range in Mo-C(Cp) distances reflects the electronic asymmetry about the molybdenum caused by the nature and spatial arrangement of the other ligands bonded to the metal. In other words, these variations are a manifestation of the trans-influence. 

Whereas d8 Ni selects the rectangular di-allylic hyperbonding pattern in (4.121), d6 Fe of ferrocene offers an additional vacant d orbital and hence opens up new geometrical possibilities of an additional cu bond. In concert with the three cu bonds and nominal sd2 (90°) hybridization (Table 4.52), the two Cp ligands are naturally expected to coordinate in r 5 (L2X) fashion to occupy the six octahedrally arrayed coordination sites of the metal. Visualization of this coordination mode is aided by considering the possible patterns of L-type (filled circles 7tcc) and X-type (half-filled circles radical) sites of L2X Cp... 

Most of the kinetic models predict that the sulfite ion radical is easily oxidized by 02 and/or the oxidized form of the catalyst, but this species was rarely considered as a potential oxidant. In a recent pulse radiolysis study, the oxidation of Ni(II and I) and Cu(II and I) macrocyclic complexes by SO was studied under anaerobic conditions (117). In the reactions with Ni(I) and Cu(I) complexes intermediates could not be detected, and the electron transfer was interpreted in terms of a simple outer-sphere mechanism. In contrast, time resolved spectra confirmed the formation of intermediates with a ligand-radical nature in the reactions of the M(II) ions. The formation of a product with a sulfonated macrocycle and another with an additional double bond in the macrocycle were isolated in the reaction with [NiCR]2+. These results may require the refinement of the kinetic model proposed by Lepentsiotis for the [NiCR]2+ SO/ 02 system (116). 

Under these conditions, the formation rate constant, k, can be estimated from the product of the outer sphere stability constant, Kos, and the water loss rate constant, h2o, (equation (28) Table 2). The outer sphere stability constant can be estimated from the free energy of electrostatic interaction between M(H20)q+ and L and the ionic strength of the medium [5,164,172,173]. Consequently, Kos does not depend on the chemical nature of the ligand. A similar mechanism will also apply to a coordination complex with polydentate ligands, if the rate-limiting step is the formation of the first metal-ligand bond [5]. Values for the dissociation rate constants, k, are usually estimated from the thermodynamic equilibrium constant, using calculated values of kf ... 

R. Merkel, P. Nassoy, A. Leung, K. Ritchie, and E. Evans Energy Landscapes of Receptor-Ligand Bonds Explored with Dynamic Force Spectroscopy. Nature 397, 50 (1999). 

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