Electron paramagnetic resonance spectra of the bis-(π-cyclopentadienyl)vanadium(IV) and bis-(π-cyclopentadienyl)niobium(IV) complexes (π-C5H5)2VX2and (π-C5H5)2NbX2

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Abstract
X-Band e.p.r. spectra of magnetically dilute glasses containing (π-C5H5)2VX2 and (π-C5H5)2NbX2(X = Cl, SCN, OCN, and CN) have been recorded at 77 and 290 K and are analysed in detail. The metal-ion spin–orbit coupling constants, ξv and ξNb, for these molecules are estimated to be +133 and +490 cm–1 respectively. Spin Hamiltonian parameters are listed for each substance and are equated to the atomic orbital coefficients in some of the molecular orbitals involved in bonding in these molecules. Except for the cyanides, in each case the unpaired electron lies essentially in a non-bonding ndz2 metal-ion orbital mixed with a small amount of the corresponding metal ion ndx2–y2 orbital, the z-axis coinciding with the C2 axis of the compound; in the cyanides this unpaired electron is delocalised into px orbitals on the ligands. π-Electrons of the cyclopentadienide rings and σ-bonding electrons derived from the other ligands X are almost completely delocalised, π-bonding to the cyclopentadienide residues being stronger than σ-bonding to the ligands X. Several redistribution complexes (π-C5H5)2NbXY have been detected. E.p.r. techniques can be used to distinguish compounds of the type (π-C5H5)2MX2 from other compounds which contain vanadium(IV) or niobium(IV). Hückel LCAO molecular orbital calculations carried out on these molecules appear to give reasonable descriptions of their ground states.

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