NO linkage isomers

In recent years, low-T photocrystallography permitted the identification of NO linkage isomers, species completely unexpected in the past. The studies to identify unusual coordination modes are rapidly extending to other small ligands such as N2, and triatomic molecules like N02 and S02. These species are potential intermediates in many relevant reactions, and their identity and properties can be also interrogated using theoretical methodologies, which eventually confirm or even predict the existence of still undetected species. 

CrNH3Cl(H20)2]Cl2. Since the Cr is and coordinated by four ligands, the complex cation is likely to be tetrahedral. There are no linkage isomers possible and no enantiomers for this complex ion. 

Once again, the trans cr-bonding alkyl or aryl ligand exerts a powerful influence on the M-N-O bond angle. Studies on transient intermediates during photolysis of Ru-NO nitrosyls have revealed different modes of binding (and dissociation) of coordinated NO at the ruthenium centers. Metastable NO linkage isomers have been observed for MNO (M = Fe, Ru, Os) and for MNO ° complexes of Ni, as well as for FeNO iron nitrosyl porphyrins [208-212]. 

In a complimentary study, Bitterwolf [128], examined the photochemistry in a triethyloctylammonium hexafluorophosphate matrix at ca. 90 K by IR spectroscopy. Irradiation > 450 nm did not yield products however, an irradiation wavelength of 400 nm led to a mixture of linkage isomers of Re-NO, i.e., in addition to the N-bound form, the O-bound and side-bound (rj2) prod-... 

For a period of about 68 years after the original work of J0rgensen, no new types of linkage isomers of metal complexes were prepared. However, in recent years many such compounds have been obtained or their presence has been detected.7 The known examples are given in Table I. An examination of the... 

As part of the work on model heme FeNO complexes, mechanistic studies on the reversible binding of nitric oxide to metmyoglobin and water soluble Fe, Co and Fe porphyrin complexes in aqueous solution, ligand-promoted rapid NO or NO2 dissociation from Fe porphyrins, reductive nitrosylation of water-soluble iron porphyrins, activation of nitrite ions to carry out O-atom transfer by Fe porphyrins, demonstration of the role of scission of the proximal histidine-iron bond in the activation of soluble guanylyl cyclase through metalloporphyrin substitution studies, reactions of peroxynitrite with iron porphyrins, and the first observation of photoinduced nitrosyl linkage isomers of FeNO heme complexes have been reported. 

Figure 27 Ortep diagrams of CpNi(NO) (a), and its side-on, r] -NO, (b) linkage isomer ...

---