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Two linear tetradentate phosphine ligands, meso -Ph 2 PCH 2 P(Ph)CH 2 X CH 2 P(Ph)CH 2 PPh 2 ( X = CH 2 ( meso -dpmppp), NBn ( meso -dpmppmNBn; Bn = benzyl)) were utilized to synthesize unsymmetrical dinuclear Rh I complexes, [Rh 2 Cl 2 ( meso -dpmppp)(L)] (L = XylNC ( 1a ), CO ( 1b )) and [Rh 2 Cl 2 ( meso -dpmppmNBn)(L)] (L = XylNC ( 1c ), CO ( 1d )), where electron-deficient Rh I → Rh I centers with 30 valence electrons are supported by a tetraphosphine in an unusual cis- / trans - P,P coordination mode. The Rh I dimers of 1a - d were treated with HCl under air to afford the Rh I → Rh III dimers with 32 e - , [Rh 2 Cl 4 ( meso -dpmppp)(L)] (L = XylNC ( 4a ), CO ( 4b )) and [Rh 2 Cl 4 ( meso -dpmppmNBn)(L)] (L = XylNC ( 4c ), CO ( 4d )), via intermediate hydride complexes, [{RhCl 2 (μ-H)RhCl(L)}( meso -dpmppp)] (L = XylNC ( 2a ), CO ( 2b )) and [{RhCl 2 (μ-H)RhCl(L)}( meso -dpmppmNBn)] (L = XylNC ( 2c ), CO ( 2d )), and [{Rh(H)Cl 2 (μ-Cl)Rh(L)} (meso -dpmppp)] (L = XylNC ( 3a ), CO ( 3b )) and [{Rh(H)Cl 2 (μ-Cl)Rh(L)} (meso -dpmppmNBn)] (L = XylNC ( 3c ), CO ( 3d )). The hydride intermediates 2 and 3 were monitored under nitrogen by 1 H{ 31 P} and 31 P{ 1 H} NMR techniques to reveal two reaction pathways depending on the terminal auxiliary ligand L. Further, the reductive dechlorination converting Rh I Rh III ( 4b , d ) to Rh I 2 ( 1b , d ) was accomplished with a CO terminal ligand by reacting with various amines that acted as one-electron reducing agents through an inner-sphere electron transfer mechanism. DFT calculations were performed to elucidate the electronic structures of 1a - d and 4a - d and to estimate the structures of the hydride intermediate complexes 2 and 3 .