Osmium , dinitrogen complexes

Osmium(II) forms no hexaaquo complex and [Os(NH3)g] +, which may possibly be present in potassium/liquid NH3 solutions, is also unstable. [Os(NH3)5N2] and other dinitrogen complexes are known but only ligands with good 7r-acceptor properties, such as CN, bipy, phen, phosphines and arsines, really stabilize Os , and these form complexes similar to their Ru analogues. 

Some bis(dinitrogen) complexes exist, generally as m-isomers (presumably this minimizes competition for the metal t2g electron density in 7r-bonding). Unlike ruthenium, osmium(III) dinitrogen complexes do exist, showing osmium(III) to be a better 7r-donor not surprisingly, they are more labile than the osmium(II) species. 

Diazadiphosphetidines nomenclature of, 14 3 synthesis of, 14 71 Diazaphosphetidinone, 14 85 1,6-Diazatriptycene, 33 1-2 Diazene, see also Diimide complexes, osmium, 37 260 ligand, 27 227-235 complexes, 27 230-235, 256-257 free state, 27 227-230 in preparation of dinitrogen complexes, 27 222, 232... 

Dinitrogen complexes of osmium are often synthetically useful because the N2 group can easily be displaced, especially by acids. 

There are also bis(dinitrogen) complexes of osmium(II). The first to be made was cis-[Os(N2)2(NH3)4]2+ by a diazotization procedure using [Os(N2)(NH3)5]2" and nitrite in acid solution.270 Vibrational spectroscopic data using both 2H and 15N substitution were obtained for the salts,271 which are useful precursors for other osmium complexes involving pyrazine (see p. 536). A similar preparative procedure involving [Os(N2)(NH3)4L]2+ gives m-[Os(N2)2(NH3)3L]2+ (L = isonicotinamide).98... 

The kinetic stability of these osmium(III) dinitrogen species, while less than that of the corresponding osmium(II) complexes, is far greater than for ruthenium(III) and shows that osmium(III) is capable of effective metal-to-ligand donation. Comparisons have been drawn, using electrochemical and pXa data, between the 7r-acceptor properties of CO and N2 in [OsL(NH3)5]3+ (L = CO, N2) species 54 thus, the pKR of [Os(NH3)6]3+ lies near 16,55 that of [Os(N2)(NH3)5]3+ is about 6.6 and that of [Os(NH3)5CO]3+ about 2.5. In basic solution osmium(III) dinitrogen complexes disproportionate to an osmium(II) dinitrogen species and osmiumfVI).64... 

Man W-L, Chen G, Yiu S-M, et al. Formation of p-dinitrogen (salen)osmium complexes via ligand-induced N---N coupling of (salen)osmium(VI) nitrides. Dalton Trans. 

Buhr JD, Taube H. Dinitrogen complexes of osmium(III) haloammines. Inorg Chem. 1980 19 2425-2434. 

The bulky ruthenium TMP complex Ru(TMP) is very electron deficient in the absence of any coordinating ligand, and a tt-complex with benzene has been proposed. In fact, it readily coordinates dinitrogen, forming the mono- and bis-N adducts Ru(TMP)(N2)(THF) and Ru(TMP)(N2)2, - As a result, the use of the TMP ligand for careful stereochemical control of the chemistry at the metal center, which has been very successful for the isolation of elusive rhodium porphyrin complexes, is less useful for ruthenium (and osmium) because of the requirement to exclude all potential ligands, including even N2,... 

---