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The geometries, energetics, and preferred spin states of the second-row transition metal tris(butadiene) complexes (C 4 H 6 ) 3 M (M = Zr–Pd) and their isomers, including the experimentally known very stable molybdenum derivative (C 4 H 6 ) 3 Mo, have been examined by density functional theory. Such low-energy structures are found to have low-spin singlet and doublet spin states in contrast to the corresponding derivatives of the first-row transition metals. The three butadiene ligands in the lowest-energy (C 4 H 6 ) 3 M structures of the late second-row transition metals couple to form a C 12 H 18  ligand that binds to the central metal atom as a hexahapto ligand for M = Pd but as an octahapto ligand for M = Rh and Ru. However, the lowest-energy (C 4 H 6 ) 3 M structures of the early transition metals have three separate tetrahapto butadiene ligands for M = Zr, Nb, and Mo or two tetrahapto butadiene ligands and one dihapto butadiene ligand for M = Tc. The low energy of the experimentally known singlet (C 4 H 6 ) 3 Mo structure contrasts with the very high energy of its experimentally unknown singlet chromium (C 4 H 6 ) 3 Cr analog relative to quintet (C 12 H 18 )Cr isomers with an open-chain C 12 H 18  ligand.

Tris(Butadiene) Compounds versus Butadiene Oligomerization in Second-Row Transition Metal Chemistry: Effects of Increased Ligand Fields