The reaction of vic-dioximes with the organonitrile platinum(IV) complexes trans-[PtC4(RCN)(2)] (R = Me, CH2-Ph, Ph, vic-dioxime = dimethylglyoxime; R = Me, vic-dioxime = cyclohexa-, cyclohepta-, and cyclooctanedione dioximes) proceeds rapidly under relatively mild conditions and affords products of one-end addition of the dioximes to the nitrile carbon, i.e. [PtCl4(NH=C(R)ON={spacer}=NOH)(2)] (1-6) (R = Me, CH2Ph, Ph, spacer = C(Me)C-(Me) for dimethylglyoxime; R = Me, spacer = C{C4H8}C, C{C5H10}C, C{C6H12}C for the other dioximes), giving a novel type of metallaligand. All addition compounds were characterized by elemental analyses (C, H, N, Cl, Pt), FAB mass spectrometry, and IR and H-1,(13){H-1}, and Pt-195 NMR spectroscopy. X-ray structure determination of the dimethylformamide bis-solvate [PtCl4(NH=C(Me)ON=C(Me)C(Me)=NOH)(2)]. 2DMF (1a) disclosed its overall trans geometry with the dimethylglyoxime part in anti configuration and the amidine one-end (rather than N,N-bidentate) coordination mode of the N-donor ligands. When a mixture of cis- and trans-[PtCl4(MeCN)(2)] in MeCN was treated with dimethylglyoxime, the formation of, correspondingly, cis-and trans-[PtCl4(NH=C(Me)ON=C(Me)C(Me)=NOH)(2)] (1) was observed and cis-to-trans isomerization in DMSO-d(6) solution was monitored by H-1, 2D [H-1,N-15] HMQC, and Pt-195 NMR spectroscopies. Although performed ab initio calculations give evidence that the trans geometry is the favorable one for the iminoacylated species [PtCl4-(ligand)(2)], the platinum(IV) complex [PtCl4(NH=C(Me)ON=C{C4H8}C=NOH)(2)] (4) was isolated exclusively in cis configuration with the two metallaligand "arms" held together by intramolecular hydrogen bonding between the two peripheral OH groups, as it was proved by single-crystal X-ray diffractometry. The classic substitution products, e.g. [PtCl2(N,N-dioximato)(2)] (12-15), are formed in the addition reaction as only byproducts in minor yield; two of them, [PtCl2(C7H11N2O2)(2)] (14) and [PtCl2(C8H13N2O2)(2)] (15), were structurally characterized. Complexes (12-15) were also prepared by reaction of the vic-dioximes with [PtCl4L(Me2SO)] (L = Me2SO, MeCN), but monoximes (Me2C=NOH, {C4H8}C=NOH, {C5H10}C=NOH, PhC(H)=NOH, (OH)C6H4C(H)= NOH) react differently adding to [PtCl4(MeCN)(Me2SO)] to give the corresponding iminoacylated products [PtCl4(NH=C(Me)ON=CRR')(Me2SO)] (7-11).