Reaction of vanadium(V)
oxide trifluoride (VOF3) and the new “naked”
fluoride reagent [(LDipp)H][F] (LDipp =
1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) leads to the isolation of [(LDipp)H][VOF4] (1) where the long
sought discrete [VOF4]− anion was finally
obtained. The neutral [(LDipp)VOF3] (2) complex was synthesized by a similar reaction between VOF3 and bulky N-heterocyclic carbene (NHC) ligand LDipp. In this context, we analyzed, by means of DFT calculations, intermolecular
interactions between [(LDipp)VOF3] (2) complexes in the crystal structure and realized that these
interactions have a significant effect on the V–Ftrans bond length. We further scrutinized ligand bonding within [(LDipp)VOF3] (2) and related complexes,
because, in this kind of complexes, a rather short distance between
CNHC and cis-halogen atoms has spurred
some discussion about the type of interactions between them. We provide
evidence of a minute π back-bonding into NHC ligands, which
is larger for chloride [(NHC)VOCl3] than fluoride
[(NHC)VOF3] complexes, although the fluoride ions
are, counterintuitively and to a larger degree, involved in back-bonding
than chloride ions. The influence of π back-bonding on V–Ftrans and V–Fcis bond lengths was also rationalized.
Finally, the hydrolysis of [(LDipp)VOF3] (2) product was studied and [(LDipp)H][VO2F2] (3) salt was obtained and characterized
as the most stable product in this system.
Can a neutral ligand
bond to a metal center of a square pyramidal
oxohalido anion at the available sixth octahedral position? Crystal
structures of some compounds indeed suggest that ligands, such as
THF, pyridine, H
2
O, NH
3
, and CH
3
CN,
can interact with the central metal atom, because they are oriented
with their heteroatom toward the metal center with distances being
within the bonding range. However, this assumption that is based on
chemical intuition is wrong. In-depth analysis of interactions between
ligands and oxohalido anions (e.g., VOX
4
–
, NbOCl
4
–
) reveals that the bonding
of a neutral ligand is almost entirely due to electrostatic interactions
between the H atoms of a ligand and halido atoms of an anion. Furthermore,
ab initio calculations indicate that the ligand–VOF
4
–
interactions represent only about one-quarter
of the total binding of the ligand within the crystal structure, whereas
the remaining binding is due to crystal packing effects. The current
study therefore shows that relying solely on the structural aspects
of solved crystal structures, such as ligand orientation and bond
distances, can lead to the wrong interpretation of the chemical bonding.
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