This article describes the differing chemical properties imparted by the two ligands, hemilabile 2-[(diisopropylphosphino)methyl]-3-methylpyridine (iPr2PN) and the related 1,2-bis(dimethylphosphino)ethane (dmpe), when attached to the 14e Cp*M(NO) scaffolds (Cp* = η5-C5Me5; M = W, Mo). For instance, the treatment of [Cp*W(NO)Cl2]2 with 2 or 1 equiv of dmpe in C6H6 affords excellent yields of [Cp*W(NO)(κ2-dmpe)Cl]Cl (1) or [Cp*W(NO)Cl2]2[μ-dmpe] (2). In contrast, the treatment of [Cp*W(NO)Cl2]2 with 1 equiv of iPr2PN in C6H6 does not produce the complex analogous to 1 but rather affords orange [Cp*W(NO)(κ2-P-N-iPr2PN)Cl][Cp*W(NO)Cl3] (3) in 90% yield. Furthermore, subsequent reduction of 1 or 2 with 2 or 4 equiv of Cp2Co in tetrahydrofuran (THF), respectively, results in the production of orange Cp*W(NO)(κ2-dmpe) (4) in good yields. However, a similar treatment of 3 with 1 equiv of Cp2Co in THF does not result in the production of Cp*W(NO)(κ2-P,N-iPr2PN), the analogue of 4, but rather generates a 1:1 mixture of the novel complexes Cp*W(NO)(H)(κ1-P-iPr2PN)Cl (5) and Cp*W(NO)(κ2-P,N-iPr2PCH-2-(3-Me-C5H3N))Cl (6), which are separable by crystallization from pentane and diethyl ether solutions, respectively. The divergent reactivity imparted by the dmpe and iPr2PN proligands is a unique demonstration of the unusual properties of a mixed-donor ligand. In the case of molybdenum, the reaction of [Cp*Mo(NO)Cl2]2 with 2 equiv of iPr2PN in C6H6 first forms Cp*Mo(NO)(κ1-P-iPr2PN)Cl2, which then converts to [Cp*Mo(NO)(κ2-P,N-iPr2PN)Cl][Cp*Mo(NO)Cl3], the analogue of 3. Reduction of the Cp*Mo(NO)(κ1-P-iPr2PN)Cl2 intermediate complex with 2 equiv of Cp2Co affords dark-green Cp*Mo(NO)(κ2-P,N-iPr2PN) (7). All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.