The carbodiimides, Ar/RNCNAr/R, (Ar = 2,6-di-isopropylphenyl (Dipp), 4-methylphenyl; R = tert-butyl, cyclohexyl, iso-propyl), react with the polar Ca–H bonds of the dimeric β-diketiminato calcium hydride, [(BDI)CaH]₂ (BDI = HC{(Me)CN-2,6-i-Pr₂C₆H₃}₂), to provide a series of heteroleptic calcium formamidinate derivatives. While reactions with carbodiimides bearing more sterically demanding substituents (Dipp, t-Bu, Cy) are straightforward, and provide four-coordinate compounds, less bulky iso-propyl or 4-methylphenyl substitution results in the sequestration of a further equivalent of the carbodiimide and the isolation of a heteroleptic hydride-bridged dinuclear species. This latter observation is suggested to be a reflection of the robust dimeric constitution of the calcium hydride reagent and the reduced steric demands of the di-para-tolyl carbodiimide reagent.