A series of ferrocenyl and thienyl mono-and biscarbene chromium(0) complexes 1-6 were synthesised.The complexes were characterised both spectroscopically and electrochemically, and the single crystal X-ray structure of 3 was determined. Electrochemical measurements in CH 2 Cl 2 revealed that the carbene double bond of 1-6 is reduced to an anion radical, − Cr-C˙at formal reduction potentials <−1.7 V vs.FcH/FcH + . A computational study on 1, 3 and 4 (B3LYP/def2-SVP level) is consistent with electrochemical results in showing that electrochemically generated chromium(I) species may be further electrochemically irreversibly oxidised to chromium(II) at E pa > 0.95 V. The reactivity towards follow-up chemical reactions of the anodically produced Cr(II) species is much higher than the reactivity of the cathodically produced radical anions as the latter was still observably reoxidised to the parent CrvC species at fast scan rates. The ferrocenyl group is oxidised electrochemically reversibly to ferrocenium at larger potentials than the electrochemically reversible oxidation of the Cr(0) centre to Cr(I). That all redox centres in 1-6 are involved in one-electron transfer steps was confirmed by comparing the ferrocenyl voltammetric wave with those of the other redox centres in linear sweep voltammetric experiments. The ferrocenyl group was electrochemically shown to stabilise the CrvC centre almost as much as the NHBu, and much more than the ethoxy and thienyl groups.

Supporting Information PlaceholderA series of ten ferrocenyl, furyl and thienyl mono-and biscarbene chromium(0) complexes were synthesized and characterized spectroscopically and electrochemically. The single crystal structure of the biscarbene complex [(CO) 5 Cr=C(OEt)-Fu'-(OEt)C=Cr(CO) 5 ] (4a) was determined: C 20 H 12 Cr 2 O 13 ; triclinic; P1; a = 6.2838(5), b = 12.6526(9), c = 29.1888(19) Å, α = 89.575(2), β = 88.030(2), γ = 87.423(2)°; Z = 4. Results from an electrochemical study in CH 2 Cl 2 were mutually consistent with a computational study in showing that the carbene double bond of 1 -6 is reduced to an anion radical, -Cr-C• at formal reduction potentials < -1.7 V vs. FcH/FcH + . The Cr centers are oxidized in two successive one electron transfer steps to Cr(II) via the Cr(I) intermediate. Only Cr(I) oxidation is electrochemically irreversible. Dicationic Cr(II) species formed upon two consecutive oneelectron oxidation processes are characterized by a peculiar bonding situation as they are stabilized by genuine CH···Cr agostic interactions. With respect to aryl substituents, carbene redox processes occurred at the lowest potentials for ferrocene derivatives followed by furan complexes. Redox process in the thiophene derivatives occurred at the highest potentials. This result is mutually consistent with a 13 C NMR study that showed the Cr=C functionality of furyl complexes were more shielded than thienyl complexes. The NHBu carbene substituent resulted in carbene complexes showing redox processes at substantially lower redox potentials than carbenes having OEt substituents.

Fischer carbene complexes of tungsten with substituents containing up to two additional different transition metals, with all the metals in electronic contact with the carbene carbon atom, were synthesised and studied both in solution and in the solid state. For the complexes of the type [W(CO) 5 {C(OR¢)R}], the substituents chosen were heteroaromatic 2-benzo[b]thienyl (2-BT), or 2-BT p-bonded to a chromium tricarbonyl fragment ([Cr(CO) 3 (2-h 6 -BT)]) or ferrocenyl (Fc) as the R-substituent, while the OR¢-substituent was systematically varied between an ethoxy or a titanoxy group, to yield the complexes 1b (R¢ = Et, R = 2-BT), 2b (R¢ = Et, R = [Cr(CO) 3 (2-h 6 -BT)]), 3b (R¢ = TiCp 2 Cl, R = 21-BT), 4b (R¢ = TiCp 2 Cl, R = [Cr(CO) 3 (2-h 6 -BT)]), 5b (R¢ = Et, R = Fc) and 6b (R¢ = TiCp 2 Cl, R = Fc). The structural features and their relevance to bonding in the multimetal carbene compounds of both these tungsten and the analogous chromium complexes were investigated as they represent indicators of possible reactivity sites in multimetal carbene assemblies. The possibility of using DFT calculations to quantify the effect of metal-containing substituents on the carbene ligands was tested and correlated with experimental parameters by employing methods such as vibrational spectroscopy, molecular orbital analysis, and cyclic voltammetry.

The series [(CO) 5 W=C(XR)Fc], 1 ( X R = O E t ) a n d 3 ( X R = N H B u ) a s w e l l a s

A series of ferrocenyl (Fc = ferrocenyl; fc = ferrocen-1,1'-diyl) and biferrocenyl (Bfc = 1',1''-biferrocenyl; bfc = 1',1''-biferrocen-1,1'''-diyl) mono-and biscarbene tungsten(0) complexes of the type [(CO) 5 W=C(OMe)R] (1, R = Fc; 3, R = Bfc) and [(CO) 5 W=C(OMe)-R'-(OMe)C=W(CO) 5 ] (2, R' = fc; 4, R' = bfc) were synthesized according to the classical synthetic methodology by reacting W(CO) 6 with LiR (R = Fc, fc, bfc), followed by a subsequent alkylation using methyl trifluoromethanesulfonate. Electrochemical investigations were carried out on these complexes to get a closer insight into the electronic properties of 1 -4. The ferrocenyl and biferrocenyl moieties in 1 -4 show reversible one electron redox events. It was further found that the Fischer carbene unit is reducible in an electrochemical one electron transfer process. For the tungsten carbonyl moieties, irreversible oxidation processes were found. In addition, charge transfer studies were performed on 1 -4 by the use of in situ UV-Vis-NIR and infrared spectroelectrochemical techniques. During the UV-Vis-NIR investigations typical low energy transitions for the mixed-valent biferrocenyl unit were found. A further observed high energy NIR absorption is attributed to a metal-metal charge transfer transition between the tungsten carbonyl fragment and the ferrocenyl/biferrocenyl group in the corresponding oxidized states, which can be described as class II systems according to Robin and Day. This assignment was verified by infrared spectroelectrochemical studies. The electrochemical investigations are supported by DFT calculations. The structural properties of 1 -4 in the solid state were investigated by single-crystal Xray diffraction studies showing no substituent effects on bond lengths and angles. The biferrocenyl derivatives exhibit synconformation of the ferrocenyl and carbene building blocks.