Platinum Metals Rev., 1968, 12, (4), 131
Nitrosyl Complexes of the Platinum Metals
In recent years much research has been carried out on platinum group metal complexes, especially those containing the carbonyl ligand. As the nitrosyl group is a nitrogen bonded ligand which in its normal mode of bonding is isoelectronic with carbon monoxide, the chemistry of complexes containing co-ordinated nitric oxide is interesting, and they may have similar potential uses to the carbonyl complexes. This article outlines our present knowledge of the nitrosyl complexes and their mode of bonding.
The nitric oxide molecule is unusual in that it has an odd number of electrons (namely 15), one of which is housed in an anti-bonding molecular orbital. It may lose this electron relatively easily to give NO+, a species isolelectronic with molecular introgen as well as carbon monoxide. Simple nitrosonium salts such as NO+ClO−4 are quite stable.
In the majority of nitrosyl complexes the ligand may be considered as NO4 with the odd electron being transferred to the metal, thus decreasing its formal oxidation state by one. The nitrosonium ion co-ordinates to the metal by donation of an electron pair in an sp orbital of the nitrogen atom completing a bond in which three electrons arc involved. This is a simplified picture of the bonding, for e.s.r. studies (1, 2) have indicated that the odd electron does spend a small amount of time on the nitrosyl ligand, and thus the true nature of the bonded ligand lies between neutral NO and NO+ although very much nearer the latter in most cases. The nitrosyl group is also a strong π -acceptor (stronger than CO) and the filled metal d orbitals will tend to overlap with the empty antibonding molecular orbital of the ligand to give the M-N bond a degree of multiple character. This is nicely demonstrated by a neutron diffraction study on Na2[Ru(NO)(OH)(NO2)4].2H2O (3). The Ru-NO bond length of 1.748Å is much shorter than might be expected for single bonding as is shown in the same complex by the Ru-NO2 bond which is 2.079Å in length. The Ru-NO2 bond is very largely of single-bond character. All the platinum metal complexes are considered to be bonded in the NO+ manner.
Nearly all nitrosyl complexes contain only one nitrosyl group and this is especially so for the platinum metals. The most likely explanation for this is that the π-acceptor ability of the ligand is so great that more than one group might remove an excessive amount of electron density from the metal. Simple binary compounds Mx(NO)y, unlike the carbonyls, are unstable, the only reported one being prepared from Ru3(CO)12 with nitric oxide at 190°C and 320 atmospheres of pressure (4). There is considerable doubt as to the composition of the red crystals formed; they may be Ru(NO)4 analogous to the iron nitrosyl, [RuNO5]n or even a carbonyl nitrosyl. No definite carbonyl nitrosyl complexes have yet been prepared, which seems surprising considering the known compounds of nickel, iron and cobalt, and investigation into such compounds may well be rewarding. The only other compounds containing more than one nitrosyl group are the chloro-bridged species Rh−1(NO)2X (X = Cl, Br, I), the tetrahedral Pd°(NO)2Cl2 and also some nitrosyl phosphine complexes of iridium. The compounds Rh−1(NO)2 X are prepared by the action of nitric oxide on the corresponding carbonyl halide Rh(CO)2X (X = Cl, Br, I) at 60°C (5, 6); they are black amorphous powders which are thought to be tetrameric. Pd(NO)2Cl2 is obtained by passing nitric oxide, laden with methanol vapour, over solid palladous chloride (7). It is unstable, evolving nitric oxide at high temperatures or in moist air. A sulphato complex Pd(NO)2SO4 is also known (7).
The mononitrosyl complexes will be considered metal by metal:
Ruthenium nitrosyl complexes are more numerous than those of any other element (over one hundred are known). They are all octahedral with the Ru-NO group having a formal charge of +3. The metal-nitrogen bond is extremely difficult to break whether the complex is anionic, neutral or cationic, and reactions normally involve exchange of the other five ligands. The formation of the metal-nitrosyl bond is so favoured that reactions of ruthenium compounds with NO, or HNO3 almost always lead to its production. A very useful starting product for many other complexes is Na2[Ru NO(OH)(NO2)4] which is prepared from ruthenium trichloride and sodium nitrite in the presence of a small amount of hydrochloric acid (8). Treatment of these yellow crystals with the appropriate hydrohalic acid yields [RuNOX5]2− (X = Cl, Br) and treatment of the latter with other ligands such as CN− and NCS− gives rise to further complexes of type [RuNOL5]2− by ligand exchange. Reaction with ammonia gives hydroxy ammines [Ru(NH3)4NO(OH)]X2 (X = Cl, Br, I); a pentammine may be prepared from either [Ru11(NH3)6]Cl2 or [Ru111(NH3)5Cl]Cl2 by reaction with sodium nitrite and hydrochloric acid. Nitrate complexes [RuNO(NO3)y(OH)3−y(H2O)2] (y = 1, 2 or 3) are produced by the action of nitric acid on the nitrite complex. These species are particularly interesting as they are extremely soluble in organic solvents and have been used to remove ruthenium by solvent extraction. Trihalo complexes [RuNOX3]n.nH2O are produced by evaporating RuO4 with hydrochloric and nitric acids. A wide range of compounds of formulae RuNOCl3L2 (L = phosphines, arsines and stibines) may be prepared from [RuNOCl3]n (9). Ruthenium (I) complexes [RuNOX2]n (X = Br, I) can be made by the action of nitric oxide at 230°C on [Ru(CO)2X2]n (10).
Osmium complexes are far less numerous than those of ruthenium, which is somewhat surprising as recent work in these laboratories has indicated that the Os-NO bond may be stronger than Ru-NO and more stable to certain types of reaction, especially reduction (11). A nitrite complex K2[OsNO(OH) (NO2)4] may be prepared by the prolonged action of potassium nitrite on K2OsCl6, and reaction of this with hydrohalic acids gives halides [OsNOX5]2− (X = Cl, Br, I) (12). An intermediate in the chloro reaction K2[OsNO(NO2)2Cl3] has recently been isolated in these laboratories (11). It seems likely that other osmium(II) nitrosyl compounds will be prepared, probably in a similar manner to the ruthenium analogues but using more extreme conditions to overcome the kinetic inertness to substitution of the third row transition metal complexes.
Very few compounds are known; the only reported ones are from the reaction of [Rh(NO)2Cl]n and phosphine type ligands giving [Rh NO L3] and [Rh NO L2Cl2] (13). It is believed that [RhNOCN5]3− is formed in the reaction of [Rh(NO)2Cl]n with alkali cyanides (14). No rhodium(III) complexes are known; the reaction of hydrochloric acid with the nitro complexes [Rh(NO2)yX 6−y]3− (y = 6, 4, 2) yields only the hexachloro compound.
The nitrosyl chemistry is again limited and the only simple complexes are the halides [Ir NOX5]− (X = Cl, Br) which are prepared by the action of hydrohalic acid on the nitro complexes [Ir(NO2)y X6−y]3− (y = 2, 3, 4 or 6) (11, 15). Although anionic, these complexes (especially the bromide) are soluble in organic solvents. Some interesting nitrosyl phosphines are also known and these contain iridium in formal oxidation states varying from II to −II (16). The action of nitric oxide on [Ir H2(PPh3)3] ClO4 gives [Ir−1(NO)2(PPh3)2] ClO4. Subsequent reaction with lithium halides gives [Ir−1(NO)2(PPh3)2X] (X = Cl, Br, I), with bromine gives [Ir11(NO)Br3 (PPh3)2] and hydrohalic acids (Ir1(NO) X2(PPh3)2] (X = Cl, Br, I). Nitric oxide with [Ir H3(PPh3)2] gives (Ir−11(NO)2PPh3].
Apart from Pd(NO)2Cl2 only a bridged polymer [PdNOCl]n and a mixed nitro-nitrato complex are known. The former is prepared by the action of nitric oxide on palladous chloride or by the interesting reaction between palladous chloride, sodium nitrite and an olefin, where the intermediate nitro complex reacts with the olefin, to produce a ketone (17).
[PdNOCl]n reacts with ammonia and cyanide ion to lose the nitrosyl ligand and form complexes [Pd(CN)4]2− and [Pd(NH3)4]2+.
K2[Pd(NO2)4(NO3)NO] is formed by the action of nitric acid on K2Pd11(NO2)4 and is a red-brown air stable substance (18).
Platinum forms a number of unstable octahedral complexes with the metal formally in the divalent state. Potassium chloroplatinite(II) reacts with nitrosyl chloride NOCl in chloroform on long shaking to give green crystals of K2[PtNOCl5] (18). A similar pentacyano compound has been made by ligand exchange. Other complexes containing ammine, nitro and nitrato ligands have been similarly prepared; [PtNO(NH3)4Cl]Cl2 may be made from NOCl and [Pt(NH3)4]Cl2, K2[PtNO(NO2)4Cl] from NOCl and K2[Pt(NO2)4] and K2[Pt NO(NO2)4NO3] from K2Pt(NO2)4 and nitric acid (18). The general instability of these compounds is not surprising as they are the only known examples of six co-ordinate platinum (II).
For further reading the following excellent reviews are recommended:
Transition Metal Nitrosyl Complexes, B. F. G. Johnson and J. A. McCleverty. Progress in Inorganic Chemistry, Vol. 7 (Interscience, 1966)
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