The coordination chemistry of the bidentate bis(imino)bis(amino)phosphate ligands [Me3SiN=P{NR}{N-(H)R}2](-), where R = n-propyl is [(LH2)-H-1](-), R = cyclohexyl is [(LH2)-H-2](-), and R = tert-butyl is [(LH2)-H-3](-), with manganese(II), is described. The bis(imino)bis(amino)phosphate-manganese(II) complexes [(eta(5)-Cp)Mn(mu-(LH2)-H-1)](2) (1), [Mn((LH2)-H-2)(2)]center dot THF (2 center dot THF), and [(eta(5)-Cp)Mn((LH2)-H-3)] (3) were synthesized by monodeprotonation of the respective pro-ligands by manganocene, Cp2Mn. The molecular structures of 1-3 reveal that the steric demands of the ligand N-substituents play a dominant role in determining the aggregation state and overall composition of the manganese(II) complexes. The coordination geometries of the Mn(II) centers are six-coordinate pseudotetrahedral in 1, four-coordinate distorted tetrahedral in 2, and five-coordinate in 3, resulting in formal valence electron counts of 17, 13, and 15, respectively. EPR studies of 1-3 at Q-band reveal high-spin manganese(II) (S = 5/2) in each case. In the EPR spectrum of 1, no evidence of intramolecular magnetic exchange was found. The relative magnitudes of the axial zero-field splitting parameter, D, in 2 and 3 are consistent with the symmetry of the manganese environment, which are D-2d in 2 and C-2v, in 3.