Structural studies metal nitrosyl complexes

A large number of paramagnetic transition metal nitrosyl complexes have been studied using electron spin resonance (ESR) spectroscopy. Information on the electronic ground state can be derived from the g-value and the hyperfine coupling constants, and many [MLslNO)]" (see Table IV) and nitrosyl porphyrin complexes (99) have been studied in this way with a view to understanding their electronic structures. 

During the last decade, the fascinating phenomena of the photochromic effect became a hot problem of molecular structure and spectroscopy due to its promising application in the design of new memory storage systems at the molecular level. This phenomenon has been discovered and studied in several nitrosyl complexes of transition metal ions [1-6]. At low temperatures, the most extensively studied sodium nitroprusside Na2[Fe(CN)5(NO)] 2H2Q (SNP) demonstrates two excited... 

A structural study of the two SO2 complexes established that the Rh-S distance for L = Cl is ca. 0.1 A shorter than for L = CO, the former being the shortest established distance among the > -pyramidal complexes. The metal atom in [Rh(ttp)(C0)(S02)] lies much closer to the plane of the four basal donor atoms than in RhCl(ttp)(S02), [RhCl(ttp)(NO)], or even Rha(ttp) This feature has been attributed to the excellent r-acceptor carbonyl, which prefers the metal in the basal plane so as to maximize 2r (CO)-

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similar effect on M-NO distances might be present also in metal nitrosyls, but the generally low precision of reported M-NO distances makes this type of analysis difficult ... 

The chemistry of transition metal nitrosyls has been reviewed, with spectra of many types used to study the electronic structure. Bonding, as described in Chapter 13, can be thought of as a linear complex of NO, isoelectronic with CO and with NO stretching frequencies of 1700 to 2000 cm or a bent complex of NO , isoelectronic with O2 and with NO stretching frequencies of 1500 to 1700 cm . The number of electrons on the metal ion and the influence of the other ligands on the metal provide for changes from one to the other during reactions. 

Enemark and Feltham s notation deliberately does not specify the oxidation state of the complex or its coordination number. However, recent studies based on independent spectroscopic and structural data may provide compelling evidence for a specific metal oxidation state, which has implications for the metal-nitrosyl bonding. Some authors therefore see a need to communicate this information. 

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