The influence of the substitution, orientation and structure of the phosphido bridges in [Fe (CO) (?-PR ) ] electrocatalysts of proton reduction has been studied. The isomers e,a-[Fe (CO) {?-P(Ar)H} ] (1a(Ar): Ar = Ph, 2?-methoxy-1,1?-binaphthyl (bn?)), e,e-[Fe (CO) {?-P(Ar)H} ] (1b(Ar): Ar = Ph, bn?) were isolated from reactions of iron pentacarbonyl and the corresponding primary phosphine, syntheses that also afforded the phosphinidene-capped tri-iron clusters, [Fe (CO) (?-CO)(?