Metalation of N-mesityl-P,P-diphenylphosphinic amide (1) in THF at room temperature with MgBu 2 and [(thf ) 2 Ca{N(SiMe 3 ) 2 } 2 ] yields [(thf ) n M{Ph 2 P(=O)-N-Mes} 2 {Ph 2 P-(=O)-NH-Mes} 2 ] [M/n = Mg/0 (2), Ca/2 (3)] regardless of the applied stoichiometry. These complexes are stabilized by weak intramolecular N-H···N hydrogen bridges. In both complexes only one NMR signal set is observed, however, for the magne- [a] Supporting Information (see footnote on the first page of this article): Experimental details, NMR spectra, temperature-dependent NMR spectra, crystallographic and refinement details.