The role of the atomic charges on the ligands and platinum(ii) in affecting the cis and trans influences in [PtXL(PPh3)2]+ complexes (X = NO3, Cl, Br, I; L = 4-substituted pyridines, amines, PPh3). A 31P NMR and DFT investigation

Abstract

One bond Pt–P coupling constants ¹J_PtP of a series of cationic complexes [PtXL(PPh₃)₂]⁺ (X = NO₃, Cl, Br, I; L = 4-Z-pyridines, Z = electron withdrawing or releasing groups, 4a–k; or X = Cl, L = NH₃, PhCH₂NH₂ and ⁱPrNH₂, 5a–c) have been used to establish the trans and cis influence sequences of X and pyridines. The crystal structure of compound 4f(BF₄) with Z = ^tBu has been resolved. In the pyridine complexes 4a–d (Z = H, variable X), both the trans and cis influence series of the anionic ligands X decrease along the same sequence I > Br > Cl > NO₃, as previously found for [PtX(PPh₃)₃]⁺ (X = NO₃, Cl, Br, I, 3a–d), however in 4a–d the cis influence turns out to be more important than the trans. On the contrary, in [PtCl(4-Z-py)(PPh₃)₂]⁺ (4b,e–k) the sequence of the trans influence of the 4-Z-pyridines is opposite to that of the cis, the latter being Z = CN > CHO > Br > PhCO > H > Me > ^tBu > NH₂, i.e. the most basic pyridine gives rise to the lowest cis influence. This correlation was found to hold also for complexes 5a–c (L = amines). All the observed trends have been fully reproduced by B3LYP/def2-SVP DFT calculations, by looking at the relevant optimized bond lengths of selected complexes of type 3, 4 and 5. Subsequent evaluation of the atomic charges, by resorting to two independent methods, i.e., the Natural Bond Order analysis of the wavefunction and the Bader's Quantum Theory of Atoms in Molecules, allowed for rationalization of the origin of the cis and trans influences. The negative charge on the nitrogen atoms of free pyridines becomes more negative upon protonation and even more so when coordinated to the [PtCl(PPh₃)₂]⁺ moiety. The least negatively charged nitrogen atom of coordinated pyridines is that of 4-CN-py (the highest cis influencing pyridine derivative), which gives rise to the lowest positive charge on Pt, confirming the relationship between the lowering of the charge on the metal ion and a high cis influence. The trans influence can be described in terms of competition between the charges on the two trans donor atoms. In contrast with the behaviour of pyridines, the positive charge on the phosphorous atom of free PPh₃ increases upon coordination to Pt(II), moreover the PPh₃ ligands acquire a substantial positive charge, thus efficiently delocalising the charge of the cationic complex.