A series of [Pd(triphosphine)(CH3CN)](BF4)2 complexes has been prepared with different chelate bites. Stoichiometric reactions of these complexes with triethylphosphine, NaBH4, and H2 have been studied, as well as the catalytic electrochemical reduction of CO2. All of these reactions show significant chelate effects. [Pd(ttpE)(CH3CN)](BF4)2 (where ttpE is bis(3-(diethylphosphino)propyl)phenylphosphine) catalyzes the electrochemical reduction of CO2 to CO in acidic dimethylformamide solutions and reacts with NaBH4 or H2 to form [Pd(ttpE)(H)](BF4). The latter complex is the decomposition product formed under catalytic conditions. X-ray diffraction studies of [Pd(ttpE)(CH3CN)](BF4)2 and [Pd(ttpE)(H)](BF4) provide insight into possible steric origins of reactivity differences between the last two complexes and analogous complexes with smaller chelate bites. [Pd(ttpE)(CH3CN)](BF4)2 has a square-planar structure with one methylene group of the ethyl substituents making a close contact with the nitrogen atom of acetonitrile. This steric interaction likely contributes to some of the reactivity differences observed. [Pd(ttpE)(H)](BF4) also has a square-planar structure with the two terminal phosphorus atoms of the triphosphine ligand distorted slightly toward the hydride ligand. Extended Hückel molecular orbital calculations suggest that small chelate bites shift electron density onto the hydride ligand, making it more hydridic, while larger chelate bites shift electron density away from the hydride ligand, making it more acidic.

etpMN) are described. The reaction of [Pd(CH3CN)4](BF4)2 with these ligands forms [Pd(triphosphine)(CH3CN)](Bp4)2 complexes that are all water soluble with the exception of [Pd(IPNetpE)(CH3-CN)](Bp4)2. The labile acetonitrile ligands are easily substituted by triethylphosphine to form [Pd(triphosphine)-( PEt3)](Bp4)2 complexes. The triethylphosphine complexes undergo quasi-reversible two-electron reductions while the corresponding acetonitrile complexes undergo two, closely spaced, irreversible, one-electron reductions. [Pd-(HOPetpE)(CH3CN)](Bp4)2 and [Pd(etpEPO)(CH3CN)](BF4)2 are catalysts for the electrochemical reduction of C02 to CO in both dimethylformamide and buffered aqueous solutions. Kinetic studies are reported for [Pd-(HOPetpE)(CH3CN)](Bp4)2 in dimethylformamide.

This article describes the preparation and spectroscopic characterization of three dicationic Ni(II) carbonyl complexes containing tripodal tetradentate ligands. An X-ray diffraction study of [Ni(PP 3 E)(CO)](BF 4 ) 2 (where PP 3 E is tris(2-(diethylphosphino)ethyl)phosphine) has been completed and confirms a trigonal bipyramidal structure with an apical carbonyl ligand. The binding of CO is weak and reversible. The stability of these complexes is enhanced by the small chelate bite imposed by the tetradentate ligands. The electrochemical behavior of these complexes depends on the nature of the donor atom trans to CO. The [Ni(NP 3 E)-(CO)] 2+ (where NP 3 E is tris(2-(diethylphosphino)ethyl)amine) cation undergoes two reversible one-electron reductions, whereas the corresponding tetraphosphine cations undergo irreversible two-electron reductions.

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