Catalyzes the insertion of Co(2+) into sirohydrochlorin. To a lesser extent, is also able to insert Fe(2+) into sirohydrochlorin, yielding siroheme. Its periplasmic location means that it cannot participate in cobalamin biosynthesis and its genomic environment suggests it is likely to be associated with a heme or metal transport system. Co-sirohydrochlorin + 2 H(+) = Co(2+) + sirohydrochlorin 2 H(+) + siroheme = Fe(2+) + sirohydrochlorin Homotetramer; dimer of dimers. Binds 1 heme b (iron-protoporphyrin IX) group per dimer, which is not required for chelatase activity but may be linked to the putative transport function. Desulfovibrio vulgaris possesses two versions of CbiK encoded within the genome, one cytoplasmic (CbiKC) and one periplasmic (CbiKP). Belongs to the CbiK family.