Ligand-Field Spectroscopy of Co(III) Complexes and the Development of a Spectrochemical Series for Low-Spin d⁶Charge-Transfer Chromophores

Abstract

A study of a series of six-coordinate Co(III) complexes has been carried out to quantify spectroscopic parameters for a range of ligands that are commonly employed to realize strong charge-transfer absorptions in low-spin, d 6 systems. Identification of any three ligand-field transitions allows for the determination of the splitting parameter (10 Dq) as well as the Racah B and C parameters for a given compound. The data revealed a relatively small spread in the magnitude of 10 Dq, ranging from ca. 23 000 cm −1 in the case of [Co(pyrro-bpy) 3 ] 3+ (where pyrro-bpy is 4,4′-dipyrrolidinyl-2,2′bipyridine) to ca. 26 000 cm −1 for [Co(terpy) 2 ] 3+ (where terpy is 2,2′:6′,2″terpyridine). Significantly, trends across the series suggest that polypyridyl ligands behave as net π-donors when interacting with Co(III), in contrast to the net π-accepting character they exhibit when bound to second-and thirdrow metals. The influence of strong σ donation associated with carbenebased ligands was evident from the data acquired for [Co(B Me ImPy) 2 ] 3+ (where B Me ImPy is 3,3′-(pyridine-2,6-diyl)bis(1-methyl-1H-3-imidazolium)), where a 10 Dq value of ca. 30 000 cm −1 was determined. Spectroscopic data were also analyzed for [Fe(bpy) 3 ] 2+ using the results on [Co(bpy) 3 ] 3+ as a reference point. A value for 10 Dq of 21 000 cm −1 was estimated, indicating a reduction in the ligand-field strength of ca. 3000 cm −1 upon replacing Co(III) with Fe(II). We suggest that this approach of taking advantage of the blueshift of the charge-transfer feature in Co(III) complexes to reveal otherwise obscured ligand-field bands can be a useful tool for the development of new ligand systems to expand the photofunctionality of first-row transition-metal-based chromophores.

Cleaner and stronger: how 8-quinolinolate facilitates formation of Co(iii)–thiolate from Co(ii)–disulfide complexes

“…13 Unexpectedly, the reaction with bpy turned out to be not straightforward, as the ligand-field strength of bpy appeared to be not as large as was anticipated. 13,30 In the current study we employed a similar strategy, but now using the bidentate ligand 8-quinolinolate. In addition to exerting a strong ligand-field this ligand is anionic in nature, which might help stabilizing the charge of the cobalt(III)-thiolate fragment.…”

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII

“…With Fe II complexes, this has been useful to approach the HOMO inversion scenario predicted computationally, leading to so-called PALCT excited states, , which can be viewed as a form of charge transfer with a particularly strong contribution of electron density from the covalent Fe–N amido interaction. Whereas the π-donor ability of amido ligands is evident, the investigation of a spectrochemical series of Co III complexes revealed that polypyridines can act as net π-donors, thus contrasting the behavior of these ligands toward second- and third-row transition metal complexes, in which they are net π-acceptors . This illustrates that well-established principles for precious metal compounds are not necessarily applicable to the first-row of transition metals, here in this case mainly because the respective metal orbital energies differ strongly .…”

“…96 This illustrates that well-established principles for precious metal compounds are not necessarily applicable to the first-row of transition metals, here in this case mainly because the respective metal orbital energies differ strongly. 96 Computational studies forecasted this effect, 119 and furthermore predicted that cyclometalating ligands could lead to long-lived MLCT states. 61,64−68 Synthetic advances now provide access to such cyclometalated Fe II complexes, 6,72,120 and in one case led to the claim of an emissive MLCT state in solution at room temperature (Figure 6d).…”

“…The energies of many MC states are however not merely a function of the ligand field parameter 10 Dq, but rather depend on the ratio of 10 Dq and B, where B is the Racah parameter characterizing the metal–ligand bond covalence. This aspect is now receiving increased attention in the design of photoactive first-row transition metal complexes, even though the nature of bonding in this compound class is usually considered more ionic in comparison to the second or third row. , Amido π-donor ligands (Figure b/Table , entry 2) help reduce the repulsion between d-electrons owing to the nephelauxetic effect, , and the Racah B parameter can be lowered markedly (Figure b) . With Fe II complexes, this has been useful to approach the HOMO inversion scenario predicted computationally, leading to so-called PALCT excited states, , which can be viewed as a form of charge transfer with a particularly strong contribution of electron density from the covalent Fe–N amido interaction.…”

Derivation of Dq/B and C/B from Electronic Spectra of Transition Metal Ions in Cubic Fields Using Auxiliary Tanabe–Sugano Diagrams

“…Nevertheless, as shown recently, the Racah parameter C plays an important role in the nephelauxetic effect and in the covalency of the metal–ligand bond for ions like Mn 4+ , Cr 3+ and Ni 2+ in several crystal hosts . Ignoring the relevance of the Racah parameter C is more dramatic when trying to explain the students the position of the spin-forbidden signals since it depends on C . − Discussion of the spin-forbidden signals in the classroom is essential for introducing the students the laser effect. , In fact, the observation of both spin-allowed and spin-forbidden d–d electronic transitions in a single complex has been implemented in the laboratory at the undergraduate level, for instance using [Cr­(oxalate) 3 ] 3– (aq) or [Al 2 O 3 :Cr 3+ ](s) . Therefore, the relevance of both Racah parameters in the nephelauxetic effect and the necessity of explaining with detail, i.e.…”