Reduction of CO₂ is one of the most important reactions that might solve the problems of global warming and shortage of fossil-fuel resources simultaneously. Metal complex catalysts are sometimes called molecular catalysts because they can be designed and synthesized on the molecular level. Some metal complexes are quite efficient and selective for CO₂ reduction. Recently, such complexes have been applied as semiconductor photocatalysts to yield hybrid metal complex/semiconductor systems. Compared to heterogeneous catalysts, metal complexes are more advantageous for elucidating reaction mechanisms. This review summarizes the reaction mechanisms that have been proposed for the photochemical CO₂ reduction reaction catalyzed by rhenium and ruthenium complexes. Rhenium complexes efficiently reduce CO₂ to selectively produce CO under various reaction conditions. On the other hand, ruthenium complexes yield CO and HCOOH, and the product selectivities are strongly dependent on the reaction conditions. Numerous reaction mechanisms have been proposed; however, no universal mechanism that can completely explain the activities and product selectivities of these catalysts exists. Why are two important intermediates, the η¹-CO₂ adduct and the hydride complex, proposed? How does the η¹-CO₂ adduct produce HCOOH? Does the hydride complex yield CO via the formate complex? What is the second electron source for the intermediate that produces CO and HCOOH? This review highlights what is already known about photocatalytic CO₂ reduction reaction mechanisms and what remains to be clarified.

减少CO ₂是最重要的反应之一，可以同时解决全球变暖和化石燃料资源短缺的问题。金属络合物催化剂有时被称为分子催化剂，因为它们可以在分子水平上进行设计和合成。一些金属络合物对于还原CO ₂相当有效且具有选择性。最近，这种配合物已被用作半导体光催化剂以产生混合金属配合物/半导体系统。与非均相催化剂相比，金属络合物在阐明反应机理方面更具优势。这篇综述总结了针对光化学CO ₂提出的反应机理rh和钌配合物催化的还原反应。various络合物有效地还原CO _2，以在各种反应条件下选择性地产生CO。另一方面，钌络合物产生CO和HCOOH，产物的选择性强烈依赖于反应条件。已经提出了许多反应机理。然而，不存在能够完全解释这些催化剂的活性和产物选择性的通用机理。为什么是两个重要的中间体，所述η ¹ -CO ₂加合物和所述氢化物配合物，提出了？如何在η ¹ -CO ₂加合物产生甲酸？氢化物配合物是否通过甲酸盐的情结？产生CO和HCOOH的中间体的第二电子源是什么？这篇综述重点介绍了有关光催化CO ₂还原反应机理的已知知识以及尚待澄清的内容。