Platinum complexes trans-influence

It is now well established in platinum(II) chemistry that Pt—Cl bonds are substantially weaker when trans to ligands such as H and CHf than when they are tram to Cl . The bonds cis to H in trans-[PtHCl(EtPh2P)2] (9) are of very similar length to those cis to Cl in tram-[PtCl2(Et3P)2] (10), so the bond weakening effect of H compared with Cl is directed specifically at the ligand trans to H . Since neither H or Cl Tt-bonds effectively with platinum, this trans influence of H must stem from its effect on the (t-electrons of the complex. 

A seminal paper [155] examined platinum-phosphorus NMR coupling constants in a series of cis- and trans-platinum(II and IV) complexes. The trans-influence had hitherto been explained in terms of d7r-p7r bonding, in other words, such a mechanism dominated with trans-effect... 

The kinetics and mechanism of ligand substitution reactions of square-planar platinum(II) dimethyl sulfoxide complexes have been exhaustively studied (173), and these workers conclude that the cis and trans influences and the trans effects of Me2SO and ethylene are similar in magnitude whereas the cis effect of Me2SO is about 100 times as large as that of ethylene. The results for reaction (5), where the stability constants, Kt, are reported to be 1.5 x 108 (L = S-Me2SO) and 4.5 x 108 (L = ethylene) corroborate this analogy (213). 

When CO is passed through a suspension of K2PtCl4 in concentrated HC1 the initial carbonyl species formed is [Pt(CO)Cl3]-. Continued reaction leads to the platinum(I) carbonyl complex [Pt2(CO)2Cl ]2-. The structure shows two PtCl2(CO)- groups linked by a short Pt—Pt bond of 2.584(2) A.309 The long Pt—Cl bond lengths indicate a large trans influence for the Pt1—Pt bond. 

Platinum(IV) isocyanide complexes PtCU(CNR)2 and [PtCl2(CNR)2(PMe2Ph)2]2+ have been prepared by the addition of Cl2 to the corresponding platinum(II) compounds (equation 118).363 It is probable that the trans influence of the isocyanide ligand on platinum(IV) is greater than that of a tertiary phosphine. 

An alkene complexed to platinum(II) is only slightly modified on coordination, but complexation to platinum(O) causes major changes. Platinum(O) alkene complexes show both weakening and lengthening of the carbon-carbon bond, as well as distortion of the plane of the double bond away from the platinum. In platinum(ll) alkene complexes the double bond lies approximately perpendicular to the square plane of platinum(II), but in platinum(O) complexes there is only a small dihedral angle between the platinum and alkenic planes. For platinum(II) the energy barrier to free rotation of the alkene about the platinum(D)-alkene bond is only about 40-65 kJ mol-1, whereas no rotation is observed with platinum(O) alkene complexes. Alkenes bonded to platinum(ll) exert a large trans effect but only have a small trans influence. 

The trans-influence series Me2S > Br > Cl > en > OH > H20 was obtained (276) from the vibrational spectra of complexes of the type [AuMe2X2]"+, and this order paralleled that found for platinum(II). In contrast to the findings for platinum(II), however, the role of 7r-bonding was suggested to be minimal. 

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