We report the reactivity of N-phosphinoguanidines of the formula (HNR)(Ph2PNR)C(NAr) (R = Pr-i and Ar = 2,6-(Pr2C6H3)-Pr-i [Dipp] for 1a, R = Pr-i and Ar = 2,4,6-Me3C6H2 [Mes] for 1b, and R = Cy and Ar = Dipp for 1c), prepared in high yields from the corresponding trisubstituted guanidines, toward main group metal alkyls AlMe3, ZnEt2, (MgBu2)-Bu-n, and (BuLi)-Bu-n to obtain novel phosphinoguanidinato and phosphinimine-amidinato compounds. Reactions of 1a-c with AlMe3 at room temperature led to the kinetic phosphinoguanidinato products [Al{kappa(2)-N,N'-(NR)C(NAr)(NRPPh2)}Me-2] (2a-c), whereas the mild heating (60-80 degrees C) of solutions of 2a-c give the thermodynamic phosphinimine-amidinato products [Al{kappa(2)-N,N'-(NR)C(NAr)(PPh2NR)}Me-2] (3a-c) after ligand rearrangement. The reactions of equimolar amounts of 1a-c and ZnEt2 initially give solutions containing unstable phosphinoguanidinato compounds [Zn{kappa(2)-N,P-(NR)C(NAr)(NRPPh2)}Et] (4a-c), which rearrange upon mild heating to the phosphinimine-amidinato derivatives [Zn{kappa(2)-N,N'-(NR)C(NAr)(PPh2NR)}Et] (6a-c). Bis(phosphinoguanidinato) compounds [Zn{kappa(2)-N,P-(NR)C(NAr)(NRPPh2)}(2)] (5a-c) can be obtained under mild conditions (<45 degrees C) in THF, whereas bis(phosphinimine-amidinato) compounds [Zn{kappa(2)-N,N'-(NR)C(NAr)(PPh2NR)}(2)] (7a-c) are also accessible under more forcing conditions (55-100 degrees C) from (i) ZnEt2 and 1b,c (2 equiv), (ii) 6a and 1a, or (iii) 5b,c. Equimolar mixtures of (MgBu2)-Bu-n and 1a-c in THF at room temperature give unstable phosphinimine-amidinato monoalkyl products [Mg{kappa(2)-N,N'-(NR)C(NAr)(PPh2NR)}Bu-n(THF)(2)] (8a-c), whereas 2 equiv of 1a,b are required to reach the bischelate compounds [Mg{kappa(2)-N,N'-(N'Pr)C(NAr)((PPh2NPr)-Pr-i)}(2)] (9a,b). Finally, phosphinoguanidinato compounds [Li{kappa(2)-N,P-(NR)C(NDipp)(NRPPh2)}(THF)(2)] (10a,c) were obtained in the reactions of 1a,c with (BuLi)-Bu-n in THF under ambient conditions. The removal of the solvent from solutions of 10a,c under partial vacuum leads to the dinuclear compounds [Li-2{mu-kappa(2)-N,N':kappa(1)-N-(NR)C(NDipp)(NRPPh2)}(2)(THF)(2)] (11a,c) after the decoordination of one of the THF molecules in 10a,c and dimerization. Heating solutions of 10a,c at 60 degrees C triggers ligand rearrangement to give phosphinimineamidinato compounds [Li{kappa(2)-N,N'-(NR)C(NDipp)(PPh2NR)}(THF)(2)] (12a,c). We also propose a mechanism for the ligand rearrangement reaction from 10a to give 12a, supported by DFT calculations, which fits nicely with our experimental results. It essentially involves a carbodiimide deinsertion reaction followed by a [3 + 2] cycloaddition between the resulting lithium phosphinoamide and the carbodiimide.