The square planar bisnitrile platinum(II) derivatives PtCl₂(NCR)₂ (R=Ph (1a); Et (1b); p-C₆H₄F (1c); p-C₆H₄^tBu (1d); m-C₆H₃Me₂ (1e); o,p-C₆H₂Me₃ (1f)) react with tetrabutylammonium hydroxide to render the monoaquo [NBu₄][trans-PtCl(HNCOR)₂(OH₂)] (2a–2f). The water molecule is σ-coordinated to platinum and the binding is reinforced by two strong hydrogen bonds to the neighboring amidate ligands (OH⋅⋅⋅OC). Substitution of water in 2a by N-donor ligands can be efficiently achieved only in the presence of a dehydrating agent as magnesium sulphate or 4 Å molecular sieves. By following this strategy, compounds [NBu₄][trans-PtCl(HNCOPh)₂(NH₂R’)] (R’=H (3), NH₂ (4), ^tBu (5a), p-C₆H₄Me (5b) have been isolated. The incoming ligands are σ-coordinated to platinum and also establish strong hydrogen bonds to the amidates (NH⋅⋅⋅OC). Treatment of 2a with halogens causes oxidation at the metal center, rendering the platinum(IV) derivatives [NBu₄][PtClX₂(HNCOPh)₂(OH₂)] (X=Cl (6a), Br (6b), I (6c)).