Vanadium complexes paramagnetism

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. 

Compound (l,3,5-Me3CgH3)V(CO)3 was proposed as an intermediate in the reaction shown in Eq. (11) but was not isolated. Treatment of compound [(l,3,S-Me3CgH3)2V][V(CO)g] with Lil in tetrahydrofuran (THE) precipitated the cation as the iodide salt. Both the neutral and cationic bisarene vanadium complexes are paramagnetic having 1 and 2 unpaired electrons, respectively 131). 

The structure elucidation was difficult because standard methods could not be utilized. The paramagnetism of amavadin caused extreme peak broadening of nuclear magnetic resonance spectra. Due to the low volatility of the vanadium complex no useful mass spectra in El, Cl or FAB mode could be obtained. ... 

Akin to group 4 metals, various NHC bidentate or tridentate ligands have been used to stabilize group 5 metals as shown in Scheme 14.24 [49,67]. Vanadium complex 44 could be cleanly oxidized (by 4-methylmorpholine Al-oxide) to the corresponding paramagnetic V (IV) oxo complex LV(=0)Cl2 [66,67]. Like the vanadium (V) complex 39, species 45 was found to be air-stable whether in the solid state or in benzene solution. The vanadium (III) complex 46 resulted from the reaction of V(NMe2)4 with the corresponding imidazolium salt and thus proceeded with a reduction to V(III). 

Vanadium, a typical transition element, displays weU-cliaractetized valence states of 2—5 in solid compounds and in solutions. Valence states of —1 and 0 may occur in solid compounds, eg, the carbonyl and certain complexes. In oxidation state 5, vanadium is diamagnetic and forms colorless, pale yeUow, or red compounds. In lower oxidation states, the presence of one or more 3d electrons, usually unpaired, results in paramagnetic and colored compounds. All compounds of vanadium having unpaired electrons are colored, but because the absorption spectra may be complex, a specific color does not necessarily correspond to a particular oxidation state. As an illustration, vanadium(IV) oxy salts are generally blue, whereas vanadium(IV) chloride is deep red. Differences over the valence range of 2—5 are shown in Table 2. The stmcture of vanadium compounds has been discussed (6,7). 

For example, in Ni(CO) nickel metal having 28 electrons coordinates four CO molecules to achieve a total of 36 electrons, the configuration of the inert gas krypton. Nearly every metal forming a carbonyl obeys the 18-electron rule. An exception is vanadium, forming a hexacarbonyl in which the number of electrons is 35. This carbonyl, which has a paramagnetism equivalent to one unpaired electron, however, readily adds one electron to form a closed valence shell complex containing the V(CO)(, anion. 

In most paramagnetic sandwich complexes the proton hf structure is not resolved in low temperature powder or single crystal EPR spectra. In vanadium dibenzene, a typical example of this type of compound, the poor resolution is due to the fact that the aromatic rings are rigidly frozen at T < 50 K and thus the proton hfs tensors of the benzene rings are no longer magnetically equivalent. 

The kinetics and stoicheiometry of the oxidation of oxovanadium(iv) by peroxide have been studied and continuous-flow e.s.r. techniques used to investigate the intermediate which appears to be a vanadium(v) complex with a paramagnetic ligand, formulated as either [OVOO,aq] or [V02(H02). aq]. Six- and seven-co-ordinate vanadium(v) has been identified by. Y-ray crystallography in the peroxo-complexes (NH4)[V0(02)2-Nh ]36o (NH4)4[0 V0(02)2 2] respcctively. and seven-co-ordination... 

A full account of the crystal structure determination of potassium heptacyano-vanadate(m) dihydrate has appeared.387 Apart from clearing up the uncertainty surrounding the nature of the vanadium(m) cyanide complex, this study has established the V(CN)7- ion as one of the very few examples of a seven-co-ordinate complex containing only simple unidentate ligands. The choice of this co-ordination number has been rationalized on the basis of the nine-orbital rule. The two electrons of the paramagnetic V(CN)4 ion (pef = 2.8 BM) occupy two separate orbitals,... 

A paramagnetic vanadium-benzyne complex (112) has been synthesized by thermolysis of CpVPh2(PMe3)2 (111) at 50°C tia = 3-4 h) [Eq. (18)].64 An X-ray crystal structure was reported for 112. Bond lengths for the benzyne ligand are given in Table III. The C1-C2 distance [1.368(5) A] is similar to those in the other structurally characterized benzyne complexes. A band in the IR spectrum of 112 at 1547 cm"1 was assigned to the coordinated triple bond. 

Components of fluidized cracking catalysts (FCC), such as an aluminosilicate gel and a rare-earth (RE) exchanged zeolite Y, have been contaminated with vanadyl naphthenate and the V thus deposited passivated with organotin complexes. Luminescence, electron paramagnetic resonance (EPR) and Mossbauer spectroscopy have been used to monitor V-support interactions. Luminescence results have indicated that the naphthenate decomposes during calcination in air with generation of (V 0)+i ions. After steam-aging, V Og and REVO- formation occurred. In the presence of Sn, Tormation Of vanadium-tin oxide species enhance the zeolite stability in the presence of V-contaminants. 

Thus, x-ray powder diffraction, electron paramagnetic resonance, luminescence and Mossbauer data suggest that a complex of Sn, V0+ and oxygen forms that leads to the passivation of vanadium when deposited on the zeolite, and on zeolite/gel mixtures. This complex may be a compound like VpSnO, or similar higher molecular weight species. Evidence of Sn/V all oy formation has not been found from Mossbauer spectroscopy. 

The compound [VCl3(terpy)] is a paramagnetic, octahedral complex formed in the reaction of VCI3 (100) or [VCl3( BuNC)3] (408) with terpy. A bis(terpy)vanadium(III) complex is presumably the product of oxidation of [V(terpy)2] (49). A red polynuclear complex is formed from the reaction of aqueous vanadium(III) solutions with one equivalent of terpy (49). 

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