Roles of the hydrogen atoms attached to the metal center of transition metal polyhydride complexes, L_nMH_x (x ≥ 3), are analyzed for about forty types of organic reactions catalyzed by such class of species. Reactions involve nearly every main organic functional group and represent friendly environmental procedures of synthesis of relevant and necessary molecules in several areas ranging from energy and environment to medicine or pharmacology. Catalysts are mainly complexes of group 8 metals, along with rhenium and iridium, and manganese and cobalt to a lesser extent. Their MH_x units can be formed by Kubas-type dihydrogen, elongated dihydrogen, or hydride ligands, which facilitate both the homolytic and heterolytic σ-bond activation reactions and hydrogen transfer processes from the metal center to unsaturated organic molecules. As a consequence of the ability of polyhydride complexes to activate σ-bonds, the vast majority of the reactions catalyzed by derivatives of this class involve at least one σ-bond activation elemental step, whereas two sequential ruptures of σ-bonds and the cross-coupling of the resulting fragments take place in a variety of reactions of C–H functionalization and hydrodefluorination. The hydrogen transfer processes usually generate highly unsaturated metal fragments, which are very reactive and extremely active in interesting C–C coupling reactions. Polyhydride complexes bearing Kubas-type dihydrogen ligands are the last intermediates in dehydrogenation processes, while they can be the first ones in hydrogenation reactions. Polyhydrides coordinating elongated dihydrogen ligands are acidic, while classical hydride complexes behave as Brønsted bases. The combination of the properties of both types of species in a catalytic cycle gives rise to interesting outer-sphere processes. The basic character of the classical hydride ligands also confers them the ability of cooperating in the coordination of acidic molecules such as boranes, which is of great relevance for reactions involving the activation of a B–H bond. Multiple bonds of unsaturated organic molecules also undergo insertion into the M–H bond of the catalysts. Such insertions are a key step in many processes.

中文翻译：

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多氢化物配合物的均相催化

分析了连接到过渡金属多氢化物配合物 L _n MH _x ( x ≥ 3) 金属中心的氢原子对此类物种催化的大约 40 种有机反应的作用。反应涉及几乎每一个主要的有机官能团，代表了从能源和环境到医学或药理学等多个领域中相关和必要分子的友好环境合成过程。催化剂主要是第 8 族金属的络合物，还有铼和铱，以及少量的锰和钴。他们的 MH _x单元可以由 Kubas 型双氢、延长的双氢或氢化物配体形成，它们促进均裂和异裂 σ 键活化反应以及从金属中心到不饱和有机分子的氢转移过程。由于多氢化物络合物具有激活 σ 键的能力，由此类衍生物催化的绝大多数反应都涉及至少一个 σ 键激活元素步骤，而 σ 键的两个连续断裂和交叉 -所得片段的偶联发生在 C-H 官能化和加氢脱氟的各种反应中。氢转移过程通常会产生高度不饱和的金属碎片，这些碎片在有趣的 C-C 偶联反应中非常活泼且非常活跃。带有 Kubas 型二氢配体的多氢化物配合物是脱氢过程中的最后中间体，而它们可能是加氢反应中的第一个中间体。多氢化物配位延长的二氢配体是酸性的，而经典的氢化物配合物表现为布朗斯台德碱。催化循环中两种类型物种的特性组合产生了有趣的外层过程。经典氢化物配体的基本特征也赋予它们在酸性分子如硼烷的配位中协同作用的能力，这与涉及 B-H 键活化的反应密切相关。不饱和有机分子的多重键也插入到催化剂的 M-H 键中。这种插入是许多过程中的关键步骤。