Enemark-Feltham notation

In recent years, several model complexes have been synthesized and studied to understand the properties of these complexes, for example, the influence of S- or N-ligands or NO-releasing abilities [119]. It is not always easy to determine the electronic character of the NO-ligands in nitrosyliron complexes thus, forms of NO [120], neutral NO, or NO [121] have been postulated depending on each complex. Similarly, it is difficult to determine the oxidation state of Fe therefore, these complexes are categorized in the Enemark-Feltham notation [122], where the number of rf-electrons of Fe is indicated. In studies on the nitrosylation pathway of thiolate complexes, Liaw et al. could show that the nitrosylation of complexes [Fe(SR)4] (R = Ph, Et) led to the formation of air- and light-sensitive mono-nitrosyl complexes [Fe(NO)(SR)3] in which tetrathiolate iron(+3) complexes were reduced to Fe(+2) under formation of (SR)2. Further nitrosylation by NO yields the dinitrosyl complexes [(SR)2Fe(NO)2], while nitrosylation by NO forms the neutral complex [Fe(NO)2(SR)2] and subsequently Roussin s red ester [Fe2(p-SR)2(NO)4] under reductive elimination forming (SR)2. Thus, nitrosylation of biomimetic oxidized- and reduced-form rubredoxin was mimicked [121]. Lip-pard et al. showed that dinuclear Fe-clusters are susceptible to disassembly in the presence of NO [123]. 

Nitroxyl (HNO/NO ) heme-model complexes ( FeNO , according to the Enemark-Feltham notation) have received special attention due to the intermediacy of nitroxyl-heme adducts in a variety of catalytic processes related to the biogeochemical cycle of nitrogen (104). For example, for the six-electron reduction of nitrite to ammonia that is catalvzed by cytochrome c nitrite reductase (ccNir), a heme FeNO complex is proposed as an intermediate (Scheme 5) (105,106). This intermediate has also been suggested for the reduction of NO to N2O by P450nor (Scheme 6) (107). Then, the isolation of a suitable FeNO heme complex that allows structural and functional characterizations will help to imderstand the reaction mechanism of ccNir and other enz5mies. 

The following limitations of the Enemark-Feltham notation have become apparent over the last 40 years ... 

Finally the Enemark-Feltham notation has not found wide applicability for poly-nitrosyl complexes. 

NO is a fascinating diatomic radical in the context of coordination chemistry due to its notorious non-innocent behavior in transition metal complexes. For example, NO adducts of ferrous iron complexes could have electronic structures that vary all the way from a Fe(I)—NO to a Fe(III)—NO extreme with the Fe(II)—NO (radical) case being intermediate. This distinction is significant, as it can be expected that NO, NO (radical), and NO will show very different reactivities. However, characterizing the exact electronic structures of transition metal nitrosyls is difficult, which led to the establishment of the famous Enemark-Feltham [Fe—NO]"" notation (the superscript x... 

In common with Enemark—Feltham, the new notation makes no attempt to define the formal charges on the nitrosyl ligand and the formal metal oxidation state but focuses attention on the geometry of the nitrosyl, the metal s coordination number and the total electron count. As De La Cruz and Sheppard have recently pointed out [26] in their extensive analysis of the vihratiOTial data for nitrosyl complexes, the great majority of them conform to 18- and 16-electron rules, and therefore, this parameter establishes whether the molecirle has a closed shell. The total electron count has important chemical implications since it indicates whether the compound is likely to undergo electrochemical conversion or nucleophilic addition in order to achieve an 18-electron configuration. 

The usual notation for the nitrosylmetalloporohyrins is MNO n after Enemark and Feltham (162). Thus, FeNO 7 stands for Fe(II)P-NO (six electrons from the 3d orbitals of Fe(II) and one electron from NO) and CoNO 8—for CoP-NO complexes. These two complexes are characterized by different M-N-0 angles of approximately 140° and 120°, respectively. 

The electron transfer series of [Fe(NO)(cyclam-ac)F (x = +2, +1, and 0) (102) (Fig. 14) that composes the FeNO " (re = 6, 7, and 8) complex series in the convenient notation suggested by Enemark and Feltham (103) is a good example that clearly shows the value of DFT calculations applied together with experimental Moss-bauer and IR spectroscopy to gain insight into the electronic structure of metal—radical complexes. 

Enemark and Feltham noted that it is quite misleading to describe all linear complexes as derivatives of NO" and all bent complexes as derivatives of NO , but did not provide a notation for indicating the M-N-O geometry and did not connect the notation to the 8 and 18 EAN rules. 

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. 

Since the vast majority of nitrosyl complexes conform to the 18- and 16-electron rules, it seems reasonably to focus the notation on these parameters rather than the modified d-electron count proposed by Enemark and Feltham. Furthermore, the more routine nature of single-crystal X-ray measurements these days and the possibility of accurately estimating the M-N-O bond angle from spectroscopic data means that this parameter can be incorporated in the notation using the short hand introduced in Table 1, i.e. 180-160° I (linear), 140-160° i (intermediate), 110-140° b (bent). 

The Feltham and Enemark and the alternative notation described above is most reliable when the ground state of the resultant nitrosyl complex is adequately described by a single determinant wave function but will show limitations when the wave function is no longer amenable to a simple Hartree-Fock analysis and electron correlation effects have to be incorporated. For transition metal complexes... 

Fe—NO complexes also exhibit unique electronic structures due to the noninnocent nature of the nitrosyl ligand. The electrons in the Fe—NO bond are highly delocahzed and thus assignment of oxidation state in these systems is challenging and resonance structures are often proposed. For example, a complex may be described as a resonance hybrid between Fe(II)-NO <-> Fe(III)—NO a total of six electrons in the Fe 3d and NO Jt orbitals. In light of the extent of electron delocalization and difficulty in assigning oxidation state, Enemark and Feltham devised the generic notation (or EF notation) MNO, where x is the total sum of the Md and... 

The similarities and differences between iron-NO and iron-oxygen bonding have been the subjects of debate, and model compounds provide important correlations between structure, spectroscopy, and redox chemistry of iron nitrosyls. The nitrosyl groups in these complexes can be described as NO S = 0), NO S = 1/2), NO S = 0), NO (S = 1), and NO S = 1/2). The associated iron ions have charges and spins such that the overall numbers of iron d electrons plus NO Jt electrons are 6-8 in most experimentally accessible model compounds. The notation for these complexes follows the Enemark and Feltham conventions [51]. The EPR visible complexes are d , or (Fe-NO) , and d , or (Fe-(NO)2). Both S = 3/2 and S = 1/2 d complexes are found. Table 2 summarizes possible electronic states that might be considered for d iron-nitrosyl complexes. Representative references to those electronic states that are experimentally observed, or calculated, are given in the table footnotes. 

The deliberately ambiguous Feltham-Enemark notation is useful because it does not matter whether the NO is linear or bent. We consider just the M(NO)j, part of the molecule and sum the number of electrons in the metal d and NO tt orbitals. For example, in [(tacn )-Fe(NO)(N3)2] (4.10), we remove the non-NO ligands as L3-type tacn and two X-type Ns to obtain Fe(NO) +. On the covalent model, neutral Fe is (f, and neutral NO has one tt electron, making 9 in all now adjusting for the 2+ ion charge of the Fe(NO)p+ fragment, we arrive at 7 valence electrons, making the complex Fe(NO) on this notation. 

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