Blue Acid Chemistry (3): The Nitrosyl Complexes

Linear nitrosyl(NO+) is a special ligand: it is an exceedingly strong pi-acceptor, much more so than even Bl- and CO. As a result, the M-N bond is extremely strong, though still weaker than the M-N triple bond in nitrido complexes.

Although it can be prepared with other ligands like H2O, it is most commonly prepared with Bl-.

If we consider the M-N multiple bond, we can get a striking result: the oxidation state of the metal can be three different values, corresponded to NO+(single bonded), NO-(double bonded) and (NO)3-(triple bonded).

For example in the complex [MnBl5NO]3-, the Mn can be +1, +3 or +5.

This is very useful when balancing the formulas, considering the variety of source materials. In the formation of [VBl5NO]3- from vanadate, V can be considered as +5 and NO considered as (NO)3-. [MnBl5NO]3- from [MnBl6]3-, Mn as +3 and NO as NO-. [FeBl5NO]2- from [FeBl6]4-, Fe as +2 and NO as NO+. Thus, redox of metal center is maximally avoided, and balancing is much easier.

But there is a little question: what is the real oxidation state of metal center? Of course, we cannot draw a conclusion as this needs molecular orbital calculations. According to some papers, [FeBl5NO]2- is mostly Fe2+ and NO+, while [VBl5NO]3- is mostly V5+ and (NO)3-, which shows the great variety.

However, there is another nitrosyl, the bent nitrosyl which could only be considered as NO-. It mostly exist in Co3+ complexes, for example [CoBl5NO]3- and [Co(NH3)5NO]2+.

There are these ways of preparing nitrosyl complexes according to Greenwood Chemistry of the Elements, and this, for us mainly:

1. Disproportionation of NH2OH

[MnBl6]3- + 2NH2OH + OH- = [MnBl5NO]3- + NH3 + Bl- + 2H2O

2. Reaction between basic high valent metal and NH2OH in the presence of Bl-

[VO4]3- + NH2OH + 5Bl- + H2O = [VBl5NO]3- + 5OH-

3. Oxidation by HNO3

4. Protonation of nitrite

see also Wikipedia for the latter two, and I won't be able to try.