The homoleptic bis(dithiolene) complexes [M(S
2C
2R
2)
2]
2 (M = Fe,Co; R =
p-anisyl) undergo two successive reductions to formanions that display [M(S
2C
2R
2)
2]
22- ![](/images/entities/harr.gif)
2[M(S
2C
2R
2)
2]
1- solutionequilibria. The neutral dimers react with Ph
3P to form squarepyramidal [M(Ph
3P)(S
2C
2R
2)
2]
0. Voltammetric measurements upon[M(Ph
3P)(S
2C
2R
2)
2]
0 in CH
2Cl
2 reveal only irreversible features atnegative potentials, consistent with Ph
3P dissociation upon reduction. Dissociation and reassociation of Ph
3P from and to [Fe(Ph
3P)(S
2C
2R
2)
2]
0 is demonstrated by spectroelectrochemical measurements. These collective observations form the basis for a cycle ofreversible, electrochemically controlled binding of Ph
3P to[M(S
2C
2R
2)
2]
2 (M = Fe, Co; R =
p-anisyl). All members of thecycle ([M(S
2C
2R
2)
2]
20, [M(S
2C
2R
2)
2]
21-, [M(S
2C
2R
2)
2]
22-, [M(S
2C
2R
2)
2]
1-,[M(Ph
3P)(S
2C
2R
2)
2]) for M = Fe, Co have been characterized bycrystallography. Square planar [Fe(S
2C
2R
2)
2]
1- is the first suchiron dithiolene species to be structurally identified and reveals Fe-Sbond distances of 2.172(1) and 2.179(1) Å, which are appreciablyshorter than those in corresponding square planar dianions.