Metal Nitrosyl and Their Derivatives

Metal Nitrosyl and Their Derivatives

Definition:-
The Compounds containing nitric oxide(NO) asNO+ ion, NO- ion or as a neutral molecules NO arecalled nitroysl compounds.

Examples:-
Simple nitroysl compounds, NO+X- (X- = F-, Cl- , Br-);(NO+)(HSO4-)NANO-

CLASSIFICATION

Nitrosyl Compounds can be classified into three types depending on whether nitric oxide (NO) is present as NO+ ion, NO- ion or as neutral NO molecule in nitrosyl compounds.

Nitrosyl Compounds Containing NO+ Groups:-

These are simple nitrosyl compounds in which N-atom of NO+ group is directly attached with the atoms or ions.

Examples:-

Nitrosyl ( nitrosonium ) hydrogen sulphate
Nitrosylsulphuric acid ( NO+)(HSO4-)
Nitrosyldisulphate (NO+)(S2O7-)
Nitrosyl halides (NO+) X- ( X= F, Cl, Br )

Since the ionization potential of NO molecule is low, it can lose one electron easily to form NO+ ion. The formation of NO+ ion can be explained by the following eq:,

N2O3 + 3H2SO4 → 2NO+ + 3HSO4- + H3O+

Nitrosyl Compounds Containing NO- Groups:-

The only known examples of the compounds of this type is sodium nitrosyl, Na(NO-).

Coordination Compounds Containing Coordinated NO+ Groups:-

Metal nitrosyl are coordinated compounds in which NO molecule is attached as NO+ ion to metal atom or ion. In these compounds the attachment of NO+ ion to the metal atom or ion takes place through N-atom. In these compounds NO+ ion which is called nitrosonium or nitrosylcation acts as a ligand. The coordinated compounds of transition metals containing NO+ ion as ligand are called metal ( metallic ) nitrosyls.

Examples of Metal Nitrosyls:-

Metal Nitrosyl Carbonyl:

Co2(CO)8 + 2NO → 2[Co(CO)3NO] + 2CO

Ni(Ph3P)2(CO)2 + 2NO → Ni(Ph3P)2(NO)2 + 2CO

Cr(CO)6 + 4NO → Cr(NO)4 + 6CO

From nitrosoniumsalts:

[Ir(CO)(PPh3)2Cl] + (NO)BF4→ [Ir(CO)(NO)(PPh3)2Cl]BF4

FromN-nitrosamides:

HMn(CO)5 + p‐tolylSO2N(Me)(NO) → Mn(CO)4(NO) + CO + p‐tolylSO2NHMe

From nitritesalts:

Na[Co(CO)4] + Na(NO2) + 2HOAc → [Co(CO)3(NO)] + CO + 2NaOAc + H2O

From nitronium (NO2+)salts:

(η5‐ C5H5)Re(CO)3+ [NO2]PF6 → [(η5‐ C5H5)Re(CO)2(NO)]PF6 + CO2

Nitrosyl Chloride a Nitrosyl Compound

Preparation:-

When aqua regia( a mixture of 1 volume of conc. HNO3 and three volumes of conc.HCl ) is warmed, an orange-yellow mixture of NOCl and Cl2 is evolved.

HNO3 + 3 HCl → 2H2O + NOCl + Cl2

Direct Preparation:-

It can be prepared by direct combination of NO and Cl2 in bright sunlight or in the presence of animal charcoal at 40 – 50 OC.

2NO + Cl2 → 2 NOCl

By action of PCl5:-

It can be prepared by the action of PCl5 on KNO2:

PCl5 + KNO2 →NOCl + POCl3 + KCl

Properties of NOCl:-

NOCl is an orange-yellow gas with suffocating odour. It can be condensed to a ruby-red liquid in a freezing mixture, which freezes to a red solid. The solid becomes lemon-yellow liquid at air temperature.

Decomposition:-

It is stable and decomposes into NOCl and Cl above 160 OC.

2NOCl + ↔2NO + Cl2

Action of Metals:-

It has no action on Au and Pt but attacks Hg and many other metals forming metallic chlorides.

2Hg + 2NOCl → Hg2Cl2 + 2NO

Formation of addition compounds:-

NOCl gives addition compounds ( adducts ) with many metal chlorides e.g.,

NOCl + 2CuCl2 → CuCl. NOCl NO+ .CuCl3-

Uses of NOCl;-

NOCl is used for knowing the presence and number of double bonds. Tilden reagent also gives an idea about the nature of carbon atoms having double bonds.

Bonding:

Most of the metal nitrosyl complexes can be viewed as derivatives of the nitrosylcation (NO+) or anion (NO−). The nitrosylcation (with a bond order of 3) is isoelectronic with carbon monoxide, thus the bonding between a nitrosyl ligand and a metal follows the same principles as the bonding in carbonyl complexes. However, the bond order of neutral nitrosyl and anion are 2.5

and 2, respectively; and therefore, in order to rationalize the nature of the bonding between metal center and the nitrosyl ligand, we must understand molecular orbital diagram for the nitrosyl ligand first.

AfterlookingatthemolecularorbitaldiagramforNO,onecanimmediatelyrecognizethedifference from the carbonyl ligand that there is one extra electron in the π*-orbital. This suggests that NO ligand can be one, two or three electron donors, depending upon the type of orbital used in bonding. If the electron present inπ*-orbitalresidesonNO,i.e.,nottransferredtothemetalcenter,nitrosylligandwouldbehaveasatwo- electron donor, and the nature of metal complex should be paramagnetic. There are somenitrosyl complexes ofironandcobaltsuchas[Fe(NO)2(CO)2]and[Co(NO)(CO)3],whichwerethoughttobederivedfromneutral NO ligand but these compounds are diamagnetic in nature; and therefore do not contain unpaired electrons. This suggests that the nitrosyl ligand is not neutral in these complexes. This assumption is also supported by the fact that the displacement of a previously attached ligand by

Any other neutral ligand in metal carbonyl nitrosyl complexes is always accompanied by the release of CO group. Moreover, there are also many complexes like [Cr(NO)(CN)5]3− and [Mn(NO)(CN)5]2−, which are actually paramagnetic and were also thoughttobehavingneutralnitrosylligand.However,inthelateryears,itwasfoundthattheunpairedelectron of the nitrosyl group is actually transferred to the metal center making NO as NO+ ligand. Thus, we can say that the coordination of neutral nitrosyl is highlyunlikely.

Structure:-

The structure of metal nitrosyls can mainly be classified into three categories; first as the metal- complex systems that contain NO as a terminal ligand only, the second one as having only bridging nitrosyl group, and the third one with nitrosyl groups with the terminal as well as bridging profile.

Metal complexes with terminal nitrosyl:-

The NO group as a monodentate ligand in metal complexes acts either as a 12-electron unit (when NO− forms a single bond with the metal center and M−N−O unit is linear), or as a 10-electron unit (when NO+ forms a multiple bonds with the metal center and M−N−O unit isbent).

Metalcomplexeswithbridgingnitrosyl:-

Insomeofthemetalnitrosylcomplexes,allNOgroupsarepresent in bridging mode. One of the common examples is givenbelow

Metal complexes with the terminal as well as bridging nitrosyl:-

In some of the metal nitrosyl complexes, NO groups are present in the terminal as well as in bridging mode. Some of the common examplesare: