| Summary: | The geometries, energetics, and preferred spin states of the second-row transition metal tris(butadiene) complexes (C<sub>4</sub>H<sub>6</sub>)<sub>3</sub>M (M = Zr–Pd) and their isomers, including the experimentally known very stable molybdenum derivative (C<sub>4</sub>H<sub>6</sub>)<sub>3</sub>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<sub>4</sub>H<sub>6</sub>)<sub>3</sub>M structures of the late second-row transition metals couple to form a C<sub>12</sub>H<sub>18</sub> 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<sub>4</sub>H<sub>6</sub>)<sub>3</sub>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<sub>4</sub>H<sub>6</sub>)<sub>3</sub>Mo structure contrasts with the very high energy of its experimentally unknown singlet chromium (C<sub>4</sub>H<sub>6</sub>)<sub>3</sub>Cr analog relative to quintet (C<sub>12</sub>H<sub>18</sub>)Cr isomers with an open-chain C<sub>12</sub>H<sub>18</sub> ligand.
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