Porphyrin Dimers Linked by Conjugated Butadiyne Bridges: Preparations, Spectra, Voltammetry and Reductive Spectroelectrochemistry of {[ M ( OEP )](μ- C 4)[ M ( OEP )]}( M 2≡ H 4, Co 2, Ni 2, Cu 2, Zn 2, Pd 2, Pt 2, Co / Ni , Ni / Cu , Ni / Zn )