The hapticity of cyclooctatetraene in its first row mononuclear transition metal carbonyl complexes: Several examples of octahapto coordination

The two cyclooctatetraene metal carbonyls that have been synthesized are the tetrahapto derivative (η4-C8H8)Fe(CO)3 and the hexahapto derivative (η6-C8H8)Cr(CO)3 using the reactions of cyclooctatetraene with Fe(CO)5 and with fac-(CH3CN)3Cr(CO)3, respectively. Related C8H8M(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 (η8-C8H8)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 η8-C8H8 ring. Thus the lowest energy structures (η8-C8H8)Ti(CO)n (n = 3, 2, 1), (η8-C8H8)M(CO)n (M = V, Cr; n = 2, 1), and (η8-C8H8)Mn(CO) all have octahapto η8-C8H8 rings. An exception is (η6-C8H8)Fe(CO), with a hexahapto η6-C8H8 ring and thus only a 16-electron configuration for the iron atom. Hexahapto (η6-C8H8)M(CO)n structures are predicted for the known (η6-C8H8)Cr(CO)3 as well as the unknown (η6-C8H8)Ti(CO)4, (η6-C8H8)V(CO)3, (η6-C8H8)Mn(CO)2, and (η6-C8H8)Fe(CO)2 with 18, 18, 17, 17, and 18 electron configurations, respectively, for the central metal atoms. There are two types of tetrahapto C8H8M(CO)n complexes. In the 1,2,3,4-tetrahapto (η4-C8H8)M(CO)n complexes two adjacent CC double bonds, forming a 1,3-diene unit similar to butadiene, are bonded to the metal atom. In the 1,2,5,6-tetrahapto (η2,2-C8H8)M(CO)3 derivatives two non-adjacent CC double bonds of the C8H8 ring are bonded to the metal atom. The known (η4-C8H8)Fe(CO)3 is a 1,2,3,4-tetrahapto complex. The unknown isomeric 1,2,5,6-tetrahapto complex (η2,2-C8H8)Fe(CO)3 is predicted to lie ∼15 kcal/mol above (η4-C8H8)Fe(CO)3. The related 1,2,5,6-tetrahapto complexes (η2,2-C8H8)Cr(CO)4, (η2,2-C8H8)Mn(CO)4, [(η2,2-C8H8)Mn(CO)3]−, (η2,2-C8H8)Co(CO)2, and (η2,2-C8H8)Ni(CO)2 are all predicted to be low-energy structures.

The cyclooctatetraene metal carbonyls C8H8M(CO)n (M = Ti, V, Cr, Mn, Fe, Co, Ni; n = 4, 3, 2, 1) have been investigated by density functional theory. The lowest energy structures (η8-C8H8)Ti(CO)n (n = 3, 2, 1), (η8-C8H8)M(CO)n (M = V, Cr; n = 2, 1), and (η8-C8H8)Mn(CO) all are predicted to have octahapto η8-C8H8 rings. Other C8H8M(CO)3 derivatives are predicted to have hexahapto, 1,2,3,4-tetrahapto, 1,2,5,6-tetrahapto, or dihapto C8H8 rings.Figure optionsDownload as PowerPoint slide