Vanadium, coordination compounds

Vanadium coordination compounds classification and analysis of crystallographic and structural data. C. E. Holloway and M. Melnik, Rev. Inorg. Chem., 1985,7, 75 (185). 

In this section, selected and representative vanadium coordination compounds will be introduced. The aim is to provide a first overview of those coordination modes which are related - or can be related - to the coordination of vanadium to biogenic ligands in its biologically relevant oxidation states +III, +IV and +V. Additional, and usually more complex, structures will be provided in those sections of Chapters 4 and 5, that are dedicated to model chemistry. For simplicity, the coordination compounds will be grouped according to the type of ligand functions dominating the coordination sphere ... 

Vanadium coordination compounds containing at least one V-C bond. 

Holloway CE, Melnik M (1985) Vanadium coordination compounds classification and... 

Coordination compounds of vanadium are mainly based on six coordination, in which vanadium has a pseudooctahedral stmcture. Coordination number four is typical of many vanadates. Coordination numbers five and eight also are known for vanadium compounds, but numbers less than four have not been reported. The coordination chemistry of vanadium has been extensively reviewed (8—12) (see Coordination compounds). 

Vanadium(V) Oxytrichloride. Vanadium(V) oxytrichloride (VOCl ) is readily hydrolyzed and forms coordination compounds with simple donor molecules, eg, ethers, but is reduced by reaction with sulflir-containing ligands and molecules. It is completely miscible with many hydrocarbons and nonpolar metal hahdes, eg, TiCl, and it dissolves sulfur. 

Vanadium(III) Chloride. Vanadium(III) chloride (vanadium trichloride, VCl ) is a pink-violet sohd, is readily hydrolyzed, and is insoluble in nonpolar solvents but dissolves in donor solvents, eg, acetonitrile, to form coordination compounds. Chemical behavior of the tribromide (VBr ) is similar to that of VCl. ... 

Although trialkyl- and triarylbismuthines are much weaker donors than the corresponding phosphoms, arsenic, and antimony compounds, they have nevertheless been employed to a considerable extent as ligands in transition metal complexes. The metals coordinated to the bismuth in these complexes include chromium (72—77), cobalt (78,79), iridium (80), iron (77,81,82), manganese (83,84), molybdenum (72,75—77,85—89), nickel (75,79,90,91), niobium (92), rhodium (93,94), silver (95—97), tungsten (72,75—77,87,89), uranium (98), and vanadium (99). The coordination compounds formed from tertiary bismuthines are less stable than those formed from tertiary phosphines, arsines, or stibines. 

Although much of the V NMR has been performed on model systems or catalytic materials containing vanadium, 29 >30 compounds such as V2O5 or VOPO4 are used in both the catalysis and lithium battery fields, and many of the results can be used to help elucidate the structures of vanadium-containing cathode materials. V NMR spectra are sensitive to changes in the vanadium coordination number and distortions of the vanadium local environments from regular tetrahedra or octahedra. >33 5>V isotropic chemical shifts of between —400 and —800 ppm are seen for vanadium oxides, and unfortunately, unlike... 

Various five-coordinate compounds are known for the configuration d of the oxovanadium(IV) and titanium(III). The complexes of the oxo-vanadium can have structures (more or less distorted) referable to the trigonal bip5n amid or more commonly to the square p n amid. 

Some tribulations in the experimental work and the variety of colours observed in early vanadium chemistry can now be explained as due to the formation of various coordination compounds, early symptoms of a rich and challenging chemistry. 

The great recent development in electrochemical techniques will certainly be helpful for the study of redox processes of a metal which can occur in so many oxidation states. Multinuclear NMR spectrometers will allow increased use of 51V resonance as a routine method for the characterization of complexes in solution. Other recent developments are the study of polynuclear complexes, metal clusters (homo and hetero-nuclear) and mixed valence complexes, and it can be anticipated that these topics will soon become important areas of vanadium coordination chemistry, although the isolation of compounds with such complex... 

Vanadium(III) compounds are usually prepared from Vv or Viv by electrolytic reduction many complexes were prepared using VC13 as starting material. Usually, vanadium(III) complexes are unstable towards air or moisture and have to be prepared under an inert atmosphere. Many have octahedral coordination and their electronic spectra are typical of d2 ions with three spin-allowed d-d transitions ... 

Bis(cyclopentadienyl)vanadium(II) complexes applications, 5, 39 coordination compounds, 5, 37 vanadocenes, 5, 36... 

Coordination compounds of vanadium(V) also catalyze peroxidative halogenation reactions where the reactive oxidant is a monoperoxo complex of a mononuclear vanadium compound (Figure 6) [11,22,92-95,99]. 

There is little information available on the complexation of alcohols by live- or six-coordinate vanadium(V) compounds. Complexes of ethanolamine do form heteroligand products with alcohols [12], The formation constants for these materials are in the order of 0.2 to 0.5 M 1 and, therefore, are not very different from similar formation constants observed for alkoxovanadates. 

Reactions of these ligands have not been studied in aqueous solution. However, their complexes are readily synthesized and are stable but reactive towards heteroligands [41,42], The reported structures all show the vanadium coordinated in monomeric units after the fashion depicted in Scheme 4.9. The multidentate thiolato complexes with tri- or tetradentate functionality are sufficient to satisfy the coordination requirements of the vanadium nucleus. Structurally, the compounds are not much different from analogous complexes formed with oxygen ligands (Section 4.4.2). 

The chemistry already described is reproduced by numerous ligands that have not specifically been addressed in the previous discussion. The V-salicylidenehydrazides (Scheme 4.18a) and related compounds provide a good example. The structure [76] of a typical complex, represented in Scheme 4.18b, is not very different from that proposed for the solution structures of dipeptide complexes (Scheme 4.17). Interestingly, other similar complexes, based on Schiff base-derived ligands, form dimeric [VO]2 core complexes (Scheme 4.1) via two long ( 2.4 A) VO bonds [2], The cyclic core is not necessary for dimer formation, and a dimer can form via a linear VOV bond [77], These complexes otherwise are not significantly different in their vanadium coordination from that depicted in Scheme 4.18b. 

Also, coordination compounds and metal carbonyls are able to undergo a PET, resulting in initiating radicals [63]. Recently investigated examples are iron chloride based ammonium salts [149], vanadium(V) organo-metallic complexes [150], and metal sulfoxide complexes [151]. However, the polymerization efficiency of some systems is only low due to redox reactions between the central metal ion and the growing polymer radical, and the low quantum yields of PET. 

In terms of local coordination, each sulfur sits at the center of a trigonal prism of vanadiums, which in turn are octahedrally coordinated by six sulfurs. The V-S distances are typical of coordination compounds and, while there is no S-S bonding, the sulfurs are in contact with each other. 

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