Spectroscopic Characterization of Nitrosyl Complexes

Comparing the Ligand Behavior of N-Heterocyclic Phosphenium and Nitrosyl Units in Iron and Chromium Complexes

“…The term “noninnocent” or “suspect” behavior of a metal coordination compound is associated with “the occurrence of situations, in which the assignment of oxidation states is not a priori obvious, because the ligands can also undergo electron transfer and thus ‘ligand oxidation state’ changes” . The NO ligand, which can formally occur as NO + , NO • , or NO – , was identified as “the simplest case of a suspect ligand”, and extensive spectroscopic and structural studies aimed at the characterization of its bonding mode and charge state have been carried out . In this context, theoretical predictions and experimental studies , revealed that a linear MNO geometry does not necessarily imply nitrosonium (NO + ) character of the ligand but may as well be compatible with a description as a nitrosyl (NO • ) or even nitroxide (NO – ) moiety.…”

Transition Metal Nitrosyls: Statistics, Charge Estimates via CDVR, and Studies of Tetradentate Chelate Diamides (pddi)M and (pddi)MNO (M = Cr, Fe, and Co)

“…The equation of charge for CO is shown in eq , where c CO is the resultant charge and ν(CO) is the average stretching frequency. Assessment of different ligands can be obtained from [M( CO ) x L y ] p complexes − ,− using eq , where p is the complex ion charge, c M c CO = ( ν false( CO false) − 2207 cm − 1 ) / 475 p = c normalM + x CO c CO + y normalL c normalL + z normalL ′ normalc normalL ′ + ... is the CDVR metal charge (for this work: Cr(+2.6), Fe(+2.0), Co(+1.6)), x CO is the number of CO ligands, c CO is the corresponding charge, and y L is the number of L, the ligand of interest. In the simplest case of CO and L, solving for c L affords the average charge of L.…”

“…The research above serves to augment the long history of the nitrosyl ligand in organometallic and inorganic chemistry and hopefully continues to dispel notions that electron counting methods of the former are electronically realistic, as plenty of previous investigators have also intimated. − ,− ,− , Reliance on valence bond theory and formal oxidation states plays into the difficulties in assessing an appropriate charge to an NO ligand that is intrinsically electronegative in comparison to its metal binder. It is probably best to consider NO as neutral or anionic when linear and anionic when bent in conforming to the estimated atomic number (EAN) rule, that is, the 18 e – rule.…”

“…A crucial example is NO ligation to transition metals, where traditional electron-counting methods implicate (NO) + when / M–N–O is linear (180–165°) and (NO) − when it is bent (140–115°). , The possibility of neutral NO (160–145°) ligation and its radical character contribute to the difficulty of assessing bonding. Nitrosyls have historically been assessed mainly via IR spectroscopy, where bonding modes (linear: 1960–1600 cm –1 ; bent: 1750–1600 cm –1 ; μ 2 -, μ 3 -bridging: 1650–1350 cm –1 ) manifest a significant overlap, as correlated with X-ray diffraction. − The ambiguities presented by NO and frustration in specifying its bonding mode(s) have led to the fairly widespread adoption of Enemark-Feltham notation, in which the sum of d-electrons and NO π*-electrons encompasses several possible configurations, bypassing assignment of a specific metal oxidation state. High energy spectroscopies have been applied more recently and help to provide an additional approach to discern binding modes. − …”

“…Bound NO, being attached to a Lewis acid in the simplest sense, is not the same as gas phase NO, and clearly (NO) + is an unrealistic limiting portrayal even though it may be a convenient e – -counting model. Researchers in nitrosyls have understood this conundrum − but often stick with convention, presumably since the lexicon of organometallic chemistry and its counting schemes are convenient.…”