Formation, paramagnetic metal complexes

Online chromatographic detection by fluorescence can add to the characterization of DOM and help to study complex formation between DOM and paramagnetic metal ions known to be effective fluorescence quenchers (Saar and Weber, 1982 Schmitt et al., 2002). 

The formation of the 17-electron paramagnetic vanadium complex is not surprising in view of the known corresponding carbonyl complex, however the 16-electron titanium derivative is unexpected in view of the ready formation of the 18-electron biscarbonyl and bistrifluorophos-phine metal complexes containing the 5-cyclopentadienyl ligand. The solid-state structure of the PF3 adduct of bis[2,4-dimethyl-(pentadienyl)]titanium has recently been determined (111) and is shown in Fig. 20. The corresponding vanadium complex is isomorphous. The metal-PF3 distances are 2.326(Ti) and 2.275(V) A. 

Connelly et al., (81) prepared a family of complexes of general formula [Cl3M NC)Mn CO)(dppm)2] (M = Mn, Co, Ni), the structures of which contain both tetrahedrally, M(II), and octahedrally Mn(II), coordinated metal ions (Fig. 12). The analogous compound with M = Fe (82) could not be obtained because of a redox process that resulted in an electron transfer from Fe(II) to Mn(II) and formation of [Cl3Fe (NC)Mn (CO)(dppm)2] with a diamagnetic low-spin Mn(I) center. This complex was oxidized by [Fe (Cp)(CpCOMe)](BF4) to yield the salt [Cl3Fe NC)Mn CO)(dppm)2](BF4), the cation of which contains two paramagnetic metal centers, namely, Fe(III) and Mn(II). 

From the valence bond point of view, formation of a complex involves reaction between Lewis bases (ligands) and a Lewis acid (metal or metal ion) with the formation of coordinate covalent (or dative) bonds between them. The model utilizes hybridization of metal s, p, and d valence orbitals to account for the observed structures and magnetic properties of complexes. For example, complexes of Pd(ll) and Pt(Il) are usually four-coordinate, square planar, and diamagnetic, and this arrangement is often found for Ni(II) complexes as well. Inasmuch as the free ion in the ground state in each case is paramagnetic (d, F), the bonding picture has to... 

Another possible application is the tagging of diamagnetic large molecules with a paramagnetic center (formation of a metal complex, for example) which can be removed after separation."... 

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