Magnetic properties of some niobium (IV) and tantalum (IV) complexes of distorted octahedral symmetry.
The treatment of paramagnetism using the approach of the crystal field theory is discussed, with special emphasis on the tetragonally distorted octahedral field. For the limiting cases when Delta >> lambda for positive and negative Delta, expressions for the susceptibility are derived. Complexes of NbX4 and TaX4 (X= Cl,Br) with pyridine, bipyridyl and tetramethylethylenediamine were prepared and their magnetic susceptibilities studied over a temperature range 90--300&deg;K. The data for all twelve complexes indicate that the Curie-Weiss law is not obeyed: Their effective moments exhibit small temperature dependence and the values at room temperature are well below the spin-only value for a single unpaired electron. Empirical fittings of mueff against kT/|lambda| curves from Figgis (34) show that (a) the sign of a Delta is reversed from that deduced on the basis of the crystal field model, (b) the axial field splittings are greater for the chlorides, and (c) Delta decreases in the order pyridine > bipyridyl > 4meen, for a given halide. The reversal in the sign of Delta is attibuted to the pi-donating effect from the halogens. (b) is accounted for by the order of the halide ions in the spectrochemical series. The lower Delta values for 4meen complexes are explained by the higher basicity of the ligand compared to pyridine and bipyridyl. Pyridine complexes have larger Delta values than the corresponding bipyridyl complexes probably because of the slightly better pi-accepting capacity of pyridine.
Source: Masters Abstracts International, Volume: 45-06, page: 3162.
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