Vanadium complexes sandwich compounds

This section describes the organovanadium compounds containing jj -cyclopentadienyl (Cp) or substituted (Cp ) ligands, which are classified into half-sandwich (see Halfsandwich Complexes), sandwich (see Sandwich Compound), bent sandwich (see Bent Metallocenes), and pseudo-triple decker derivatives. Cyclopentadienyl vanadium compounds are nsed for the synthesis of varions vanadium compounds as mentioned above. Generally, the oxidation state of the cyclopentadienyl vanadium complexes is Irom — 1 to -1-5. 

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. 

Elements of the first transition series from vanadium to nickel form stable bis(7j-cyclopentadienyl) complexes having the normal ir-sandwich structure typical of ferrocene. In contrast, bis(Tj-cyclopentadienyl)ti-tanium, (i7-CsH5)2Ti (1), is not even known as a discrete compound. 

The metal vapor ligand codeposition method has extended the array of such sandwich complexes. A wide range of substituents can be tolerated on the arene ring unusual arene M complexes have been prepared. Elschenbroichmade the interesting compound bis(j7 -phosphabenzene)vanadium, which is less air sensitive than the ligand itself. It turned out that the phosphorus heteroatom has a much stronger influence on the electron affinity of the free arene than of the bound arene. 

The lack of stability of terminal oxo compounds of the heavier Group 5 metals led us naturally to the door of the isoelectronic imido (NR) ligand, where the availability of a substituent attached to the multiply-bonded group would allow both steric and electronic modulation of the products stability and reactivity. There had been a handful of half-sandwich imido complexes of the Group 5 metals synthesised by other workers, especially for vanadium and tantalum, but at that time none were known for niobium. A half-sandwich imido compound of niobium we considered, therefore, a prime target. 

Cationic molybdenum, tungsten, chromium, and vanadium compounds disproportionate to give sandwich complexes M(arene)2 ... 

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