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Co(dppe)2] would be expected to have a Z)M structure lying between square planar and tetrahedral. In contrast, PP3 would be expected to impose a pyramidal structure on [Co(PP3)] as is observed for [Ni(PP3)](C104).21 Since the pyramidal geometry is energetically less favored than a Z)M geometry, the corresponding reduction would be expected to be more difficult.For nickel(II), the PP3 ligand stabilizes a five-coordinate trigonal-bipyramidal structure, while dppe and PP2 form fourcoordinate square-planar complexes. For all three nickel complexes three oxidation states are available in the potential range of -0.7 to -1.3 V vs. ferrocene, and all exhibit a reversible or quasi-reversible one-electron wave for the Ni(II/I) couple. The Ni(I/0) couple is reversible for [Ni(dppe)2](BF4)2 and irreversible for [Ni(PP3)(CH3CN)](BF4)2 and [Ni(PP2)(CH3CN)](BF4)2.For [Ni(PP3)(CH3CN)](BF4)2 the irreversibility of the Ni(I/0) couple is due to a ring opening process in which a metal-phosphorus bond is broken and a Ni(0) dimer, [Ni(PP3)]2, is formed. The rupture of the nickel-phosphorus bond is attributed to ring strain in the transient pyramidal [Ni(PP3)J complex. In contrast, [Ni(dppe)2](BF4)2 is free to distort to a tetrahedral structure on reduction, and the Ni(I/0) couple is reversible. For [Ni(PP2)-(CH3CN)](BF4)2 the irreversibility of the Ni(I/0) couple probably arises from loss of acetonitrile on reduction to Ni(0).