Abstract
The geometries, energetics, and preferred spin states of the second-row transition metal tris(butadiene) complexes (C4H6)3M (M = Zr–Pd) and their isomers, including the experimentally known very stable molybdenum derivative (C4H6)3Mo, 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 (C4H6)3M structures of the late second-row transition metals couple to form a C12H18 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 (C4H6)3M 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 (C4H6)3Mo structure contrasts with the very high energy of its experimentally unknown singlet chromium (C4H6)3Cr analog relative to quintet (C12H18)Cr isomers with an open-chain C12H18 ligand.
Highlights
Butadiene transition metal chemistry dates back to the 1930 discovery of butadiene iron tricarbonyl by Reihlen et al [1], its subsequent reinvestigation by Hallam and Pauson in 1958 [2] after the 1951 discovery of ferrocene [3,4], and its structural elucidation by Mills and Robinson [5] using X-ray crystallography
The four lowest-energy (C4 H6 )3 Pd structures are all singlets with hexahapto straightchain C12 H18 ligands of various types, leaving one uncomplexed C=C double bond (Figure 2 and Table 1)
This corresponds to a 16-electron configuration for the central palladium atom
Summary
Butadiene transition metal chemistry dates back to the 1930 discovery of butadiene iron tricarbonyl by Reihlen et al [1], its subsequent reinvestigation by Hallam and Pauson in 1958 [2] after the 1951 discovery of ferrocene [3,4], and its structural elucidation by Mills and Robinson [5] using X-ray crystallography. C12 H18 ligand and cyclo-(η2,2,2 -C12 H18 )Ni with a complexed 1,5,9-cyclododecatriene ligand (Figure 1). The latter intermediate has been structurally characterized by X-ray crystallography [6]. The bonding of the central C=C double bond of the acyclic C12 H18 ligand to the nickel atom in the former intermediate is uncertain since our recent theoretical studies [7] suggest a η3,3 -hexahapto ligand rather than the originally proposed η3,2,3 -octahapto ligand. No stable homoleptic (C4 H6 )n Ni (n = 2, 3) intermediates with unsubstituted separate butadiene ligands were observed in this system, even though the nickel atom in a hypothetical (C4 H6 ) Ni with two tetrahapto butadiene ligands would have the favored 18-electron configuration
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