The d4 halide complexes [MX(CO)(η-RCCR)Tpâ²] X = F, Cl, Br or I; R = Me or Ph; M = Mo or W; Tpâ² = hydrotris(3,5-dimethylpyrazolyl)borate} undergo one-electron oxidation to the d3 monocations [MX(CO)(η-RCCR)Tpâ²]+, isolable for M = W, R = Me. X-Ray structural studies on the redox pairs [WX(CO)(η-MeCCMe)Tpâ²]z (X = Cl and Br, z = 0 and 1), the ESR spectra of the cations [WX(CO)(η-RCCR)Tpâ²]+ (X = F, Cl, Br or I; R = Me or Ph), and DFT calculations on [WX(CO)(η-MeCCMe)Tpâ²]z (X = F, Cl, Br and I; z = 0 and 1) are consistent with electron removal from a HOMO (of the d4 complexes) which is Ï-antibonding with respect to the WâX bond, Ï-bonding with respect to the WâC(O) bond, and δ-bonding with respect to the WâCalkyne bonds. The dependence of both oxidation potential and ν(CO) for [MX(CO)(η-RCCR)Tpâ²] shows an inverse halide order which is consistent with an ionic component to the MâX bond; the small size of fluorine and its closeness to the metal centre leads to the highest energy HOMO and the lowest oxidation potential. In the cations [MX(CO)(η-RCCR)Tpâ²]+ electronegativity effects become more important, leading to a conventional order for Cl, Br and I. However, high MâF Ï-donation is still facilitated by the short MâF distance.
                                    d4 卤化物配合物 [MX(CO)(η-RCCR)Tp'] X = F, Cl, Br 或 I; R = Me 或 Ph; M = MO 或 W; Tp' = 羟基三(
3,5-二甲基吡唑基)
硼酸酯} 经过单电子氧化转化为 d3 单阳离子 [MX(CO)(η-RCCR)Tp']+,可分离得到 M = W, R = Me 的情况。关于氧化还原对 [WX(CO)(η-MeCCMe)Tp']z (X = Cl 和 Br, z = 0 和 1) 的 X 射线结构研究,阳离子 [WX(CO)(η-RCCR)Tp']+ (X = F, Cl, Br 或 I; R = Me 或 Ph) 的 ESR 光谱,以及 [WX(CO)(η-MeCCMe)Tp']z (X = F, Cl, Br 和 I; z = 0 和 1) 的 DFT 计算结果一致表明,电子从与 W-X 键的 π 反键,W-C(O) 键的 π 键,以及 W-C炔基键的 δ 键相关的 HOMO(d4 配合物)中移除。[MX(CO)(η-RCCR)Tp'] 的氧化电位和 ν(CO) 的依赖关系显示了相反的卤素顺序,这与 M-X 键具有离子成分一致;
氟原子尺寸小且靠近
金属中心,导致能量最高的 HOMO 和最低的氧化电位。在阳离子 [MX(CO)(η-RCCR)Tp']+ 中,电负性效应变得更为重要,导致 Cl、Br 和 I 的常规顺序。然而,由于短 M-F 距离,高 M-F π 供电子依然容易进行。