Roles of Bridging Ligand π-Acidity in Determining the Nature of the Lowest Excited States of Binuclear Complexes of Nickel(0). Ligand-Centered (M2→μ-LCT), Metal-Centered (μ-L → M2CT), and Intraligand (IL) Lowest Excited States for the Series of Complexes Ni2(M-L)(CNMe)2(PPh2CH2PPh2)2(L = CNR, CNMe(R)+, NO+)

K. S. Ratliff, D. L. DeLaet, J. Gao, P. E. Fanwick, C. P. Kubiak

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Abstract

The spectroscopy and photochemistry of the series of binuclear Ni(O) complexes Ni2(μ-L)(CNMe)2(dppm)2(dppm = bis(di-phenylphosphino)methane) with either bridging isocyanide ligands (L = CNMe (1), CNC6H5(2), CN-p-C6H4Cl (3), CN-p-C6H4Me (4)), bridging aminocarbyne ligands (L = CNMe(C5Hw)+(5), CNMe2+(6), CNMe(CH2Ph)+(7), CNMe(H)+(8), CNMe(Ph)+(9)), or bridging NO+(10), have been examined. Bridging-ligand substituent effects and solvent dependence of the lowest energy electronic absorption bands of the isocyanide complexes, 1–4, have been interpreted in terms of dimetal to bridging ligand charge transfer (M2→ μ-LCT). This assignment is supported by results of extended Hückel calculations, which indicate a LUMO of predominantly μ-isocyanide π* character. Bridging-ligand substituent effects on the lowest energy electronic absorption spectral bands of the related aminocarbyne complexes, 5–9, indicate that the direction of charge transfer is reversed compared to the case of the isocyanide complexes. The lowest excited states of the aminocarbyne complexes, 5–9, are assigned to bridging ligand to dimetal charge transfer (μ-L → M2CT). This assignment is supported by extended Hiicke) calculations, which indicate a LUMO of predominantly Ni-Ni antibonding character for the aminocarbyne complexes. The μ-nitrosyl complex [Ni2(μ-NO)(CNMe)2(dppm)2][PF6] (10) has also been studied. The molecule possesses a structure similar to 1 and 8. Photochemical reactivity as well as extended Hückel calculations indicates an intraligand (IL) lowest excited state for 10. Only the M2→ μ-LCT excited state of the μ-isocyanide complexes was found to participate in bimolecular photochemistry. Photolysis of I in the presence of C02(1 atm) leads to the [2 + 2] photocycloaddition of C02to the μ-CNMe ligand of 1 and the complex Ni2(μ-CN(Me)C (0)0)(CNMe)2(dppm)2(11). The degree of metal-metal interaction in complexes 1, 8, and 10 was found to be related to the Lewis acidity of the bridging ligand. Extended Hückel calculations indicate that the decreasing π* energies for the series of bridging ligands CNMe, CN(H)Me+, and NO+decrease the ground-state contribution of a metal-metal dπ* orbital, resulting in increased Ni-Ni interactions. The relative μ-L π* and Ni-Ni dπ* energies also control the nature of lowest excited states as M2→ μ-LCT, μ-L → M2CT, and IL for μ-L = CNR, CNMe(R)+, and NO+, respectively.

Original languageEnglish (US)
Pages (from-to)4022-4027
Number of pages6
JournalInorganic chemistry
Volume29
Issue number20
DOIs
StatePublished - Jan 1 1990

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