What are transition metal centers?

Some articles on transition metal centers, centers:

Iron–sulfur World Theory - Origin of Life - Pioneer Organism
... structure of a mineral base with catalytic transition metal centers (predominantly iron and nickel, but also perhaps cobalt, manganese, tungsten and zinc) ... The catalytic centers catalyzed autotrophic carbon fixation pathways generating small molecule (non-polymer) organic compounds from inorganic gases (e.g ... The catalytic transition metal centers became autocatalytic by being accelerated by their organic products turned ligands ...

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