The two cyclooctatetraene metal carbonyls that have been synthesized are the tetrahapto derivative (η 4-C 8H 8)Fe(CO) 3 and the hexahapto derivative (η 6-C 8H 8)Cr(CO) 3 using the reactions of cyclooctatetraene with Fe(CO) 5 and with fac-(CH 3CN) 3Cr(CO) 3, respectively. Related C 8H 8M(CO) n (M = Ti, V, Cr, Mn, Fe, Co, Ni; n = 4, 3, 2, 1) species have now been investigated by density functional theory in order to explore the scope of cyclooctatetraene metal carbonyl chemistry. In this connection, the existence of octahapto (η 8-C 8H 8)M(CO) n species is predicted as long as the central metal M does not exceed the 18-electron configuration by receiving eight electrons from the η 8-C 8H 8 ring. Thus the lowest energy structures (η 8-C 8H 8)Ti(CO) n ( n = 3, 2, 1), (η 8-C 8H 8)M(CO) n (M = V, Cr; n = 2, 1), and (η 8-C 8H 8)Mn(CO) all have octahapto η 8-C 8H 8 rings. An exception is (η 6-C 8H 8)Fe(CO), with a hexahapto η 6-C 8H 8 ring and thus only a 16-electron configuration for the iron atom. Hexahapto (η 6-C 8H 8)M(CO) n structures are predicted for the known (η 6-C 8H 8)Cr(CO) 3 as well as the unknown (η 6-C 8H 8)Ti(CO) 4, (η 6-C 8H 8)V(CO) 3, (η 6-C 8H 8)Mn(CO) 2, and (η 6-C 8H 8)Fe(CO) 2 with 18, 18, 17, 17, and 18 electron configurations, respectively, for the central metal atoms. There are two types of tetrahapto C 8H 8M(CO) n complexes. In the 1,2,3,4-tetrahapto (η 4-C 8H 8)M(CO) n complexes two adjacent C C double bonds, forming a 1,3-diene unit similar to butadiene, are bonded to the metal atom. In the 1,2,5,6-tetrahapto (η 2,2-C 8H 8)M(CO) 3 derivatives two non-adjacent C C double bonds of the C 8H 8 ring are bonded to the metal atom. The known (η 4-C 8H 8)Fe(CO) 3 is a 1,2,3,4-tetrahapto complex. The unknown isomeric 1,2,5,6-tetrahapto complex (η 2,2-C 8H 8)Fe(CO) 3 is predicted to lie ∼15 kcal/mol above (η 4-C 8H 8)Fe(CO) 3. The related 1,2,5,6-tetrahapto complexes (η 2,2-C 8H 8)Cr(CO) 4, (η 2,2-C 8H 8)Mn(CO) 4, [(η 2,2-C 8H 8)Mn(CO) 3] −, (η 2,2-C 8H 8)Co(CO) 2, and (η 2,2-C 8H 8)Ni(CO) 2 are all predicted to be low-energy structures.