Structural consequences of the one-electron reduction of d⁴[Mo(CO)₂(η-PhCCPh)Tp′]⁺and the electronic structure of the d⁵radicals [M(CO)L(η-MeCCMe)Tp′] {L = CO and P(OCH₂)₃CEt}

Abstract

Reduction of [M(CO)2(eta-RC[triple bond]CR')Tp']X {Tp' = hydrotris(3,5-dimethylpyrazolyl)borate, M = Mo, X = [PF6]-, R = R' = Ph, C6H4OMe-4 or Me; R = Ph, R' = H; M = W, X = [BF4]-, R = R' = Ph or Me; R = Ph, R' = H} with [Co(eta-C5H5)2] gave paramagnetic [M(CO)2(eta-RC[triple bond]CR')Tp'], characterised by IR and ESR spectroscopy. X-Ray structural studies on the redox pair [Mo(CO)2(eta-PhC[triple bond]CPh)Tp'] and [Mo(CO)2(eta-PhC[triple bond]CPh)Tp'][PF6] showed that oxidation is accompanied by a lengthening of the C[triple bond]C bond and shortening of the Mo-C(alkyne) bonds, consistent with removal of an electron from an orbital antibonding with respect to the Mo-alkyne bond, and with conversion of the alkyne from a three- to a four-electron donor. Reduction of [Mo(CO)(NCMe)(eta-MeC[triple bond]CMe)Tp'][PF6] with [Co(eta-C5H5)2] in CH2Cl2 gives [MoCl(CO)(eta-MeC[triple bond]CMe)Tp'], via nitrile substitution in [Mo(CO)(NCMe)(eta-MeC[triple bond]CMe)Tp'], whereas a similar reaction with [M(CO){P(OCH2)3CEt}(eta-MeC[triple bond]CMe)Tp']+ (M = Mo or W) gives the phosphite-containing radicals [M(CO){P(OCH2)3CEt}(eta-MeC[triple bond]CMe)Tp']. ESR spectroscopic studies and DFT calculations on [M(CO)L(eta-MeC[triple bond]CMe)Tp'] {M = Mo or W, L = CO or P(OCH2)3CEt} show the SOMO of the neutral d5 species (the LUMO of the d4 cations) to be largely d(yz) in character although much more delocalised in the W complexes. Non-coincidence effects between the g and metal hyperfine matrices in the Mo spectra indicate hybridisation of the metal d-orbitals in the SOMO, consistent with a rotation of the coordinated alkyne about the M-C2 axis.

The d4/d3redox pairs [MX(CO)(η-RCCR)Tp′]z(z = 0 and 1): structural consequences of electron transfer and implications for the inverse halide order

“…The agreement between the two approaches here is noticeably better than in the related compounds [M(CO)L(MeC≡CMe)Tp ] {L = CO or P(OCH 2 ) 3 CEt}. 2 The inverse halide order, observed by both IR spectroscopy and electrochemistry for 1-4, 6-13 and 15-18 has previously been attributed to a variation in X-to-M p p -d p donation. However, more recent detailed studies 6 of the oxidation of [FeX(dppe)(g-C 5 Me 5 )] (X = F, Cl, Br, I, H or Me) suggest an alternative explanation, namely that the HOMO energy is not related to the p-donor ability of X but is based on an ionic component to the M-X bond.…”

“…The oxidation of the (formal) d 4 neutral complexes to the d 3 cations therefore contrasts markedly with the reversible reduction of the parent dicarbonyl d 4 complexes [M(CO) 2 (g-RC≡CR)Tp ] + to d 5 [M(CO) 2 (g-RC≡CR)Tp ]. 2 The tungsten complexes 10-18 also show a second oxidation wave, between 1.49 and 1.67 V, its irreversibility implying that the d 2 -complex [WX(CO)(g-RC≡CR)Tp ] 2+ is unstable under the electrochemical conditions. For 10-17 this second oxidation is accompanied by a reversible wave, in the range −0.04 to 0.24 V, due to the reduction of a secondary product, [WFX(g-RC≡CR)Tp ] + , which will be described elsewhere.…”

The effect of redox-active cyanomanganese(I) ligands on intramolecular electron transfer to, and alkyne alignment in, M(CO)(RCCR)Tp′ (M = Mo or W) units

“…The C∫C carbons are observed as highly deshielded peaks, at d 206-220, characteristic of an alkyne behaving as a four-electron donor. 7 The cyclic voltammogram (CV) of the cyanide complex [Mn(CN)(CNXyl)(NO)(h-C 5 Me 5 )] 1 shows a partially chemically reversible oxidation wave (i red /i ox = 0.74 and 0.91 at scan rates of 200 and 1000 mV s -1 respectively), centred at 0.88 V (vs. sce) and accompanied by two small, apparently reversible, product 3 is at a much more positive potential (by ca. 1.1 V).…”

“…with CO rather than X in [MX(CO)(RC∫CR)Tp¢] (M = Mo or W, X = Cl or Br), 2 but that alignment can also depend on redox state, even when L = L¢. Thus, although the alkyne bisects the OC-Mo-CO angle in both [Mo(CO) 2 (PhC∫CPh)Tp¢] (formally d 5 ) and [Mo(CO) 2 (PhC∫CPh)Tp¢] + (formally d 4 ) 3 it moves from a similar arrangement in [WX 2 (RC∫CR)Tp¢] + (X = Cl or Br, formally d 2 ) to one in which the alkyne is more closely aligned with one W-X bond in [WX 2 (RC∫CR)Tp¢] (formally d 3 ). 4 We have now synthesised two series of complexes in order to investigate the effects of incorporating an ancillary redoxactive unit on the properties of the metal-alkyne unit.…”

The d3/d2alkyne complexes [MX2(η-RCCR)Tp′]z(X = halide, z = 0 and 1+): final links in a d6–d2redox family tree

“…The discovery of the one-electron oxidation of [Cr(CO) 2 (h-RC∫CR)(h 6 -arene)] and the one-electron reduction of [M(CO) 2 (h-RC∫CR)Tp¢] + , and the consequent structural characterisation of the redox pairs [Cr(CO) 2 (h-PhC∫CPh)(h 6 -C 6 Me 5 H)] z (z = 0, d 6 and 1+, d 5 ) 1 and [Mo(CO) 2 (h-PhC∫CPh)Tp¢] z (z = 0, d 5 and 1+, d 4 ), 2 enabled us to show that stepwise electron removal from a (formally) d 6 complex converted the coordinated alkyne from a two-to a three-to a four-electron donor. Halide substitution of one carbonyl ligand of [M(CO) 2 (h-RC∫CR)Tp¢] + then gave [MX(CO)(h-RC∫CR)Tp¢] (M = Mo or W, X = halide) which was oxidised to the isolable d 3 cation [MX(CO)(h-RC∫CR)Tp¢] + , again structurally characterised (M = W, X = Cl or Br).…”