3.8 Complexes of Palladium(II) and Platinum(II)
3.8.11 Spectroscopic Evidence for trans-
Study of a series of complexes /ra^-Pt(PEt3)2HX shows a pronounced dependence of MPt-H) upon the trans-ligand (Table 3.18).
Similarly, in complexes PtL2Cl2, the Pt-Cl stretching frequency is relatively insensitive to L in the fr<ms-isomer but shows considerable depen- dence in the cw-isomer (Table 3.19) [10O].
NMR evidence
Table 3.18 shows how the position of the low-frequency hydride resonance is affected by the trans-ligand, while study of a series of complexes trans- [Pt(PMe2Ph)2(Me)L]+ and neutral ^w-Pt(PMe2Ph)2(Me)X shows the trans-influence of the ligand on 2J(195Pt-1H) with better donors tending to reduce the value of / (Table 3.20) [152].
Platinum ammine complexes have been a fertile area for studying trans- influence. Table 3.21 lists data for a range of ammines showing how
1J(195Pt-15N) depends upon the trans-atom [153]. (A further selection of data can be found in: R.V. Parish, NMR, NQR, EPR and Mossbauer Spectroscopy in Inorganic Chemistry, Ellis-Horwood, Chichester, 1991, pp.
76, 87.) Possibly the most detailed study (of complexes of tribenzylphosphine) examined over a hundred neutral and cationic complexes [154] (Table 3.22).
Table 3.19 i/ (Pt-Cl) in complexes PtL2Cl2 (cm"1)
L NH3 PEt3 Et2S py AsEt3
trans-Isomer 331.5 340 342 342.6 339 cw-Isomer 326 303,281 330,318 343,328 314,287.5 Average 326 292 324 335.5 301
Table 3.20 The rrarcs-influence of ligands on
2J(195Pt-1H) in trans-A+ and trans-AXa
L X J (Hz)
SbPh3 55
PMe2Ph 57
P(OPh)3 58
PPh3 60
CO 63 Py 74 PhCN 79 Cl 85 Br 83 I 80
"AiSPt(PMe2Ph)2 MeL
Putting the ligands in order of their effect on the value of 6, the position of the hydride resonance, gives H > CO, PR3 > CN > tu > NO2 > SMe2 >
SCN, I > Br > Cl > NH3, py. A not dissimilar order is found for their effect on v(Pt-H) in the IR spectrum (H > CN > PR3 > I > SCN ~ NO2 ~ tu - CO - Cl > SMe2 > Br > NH3 > py). Data correlate well with those for other tertiary phosphines, e.g. Pcy3.
A seminal paper [155] examined platinum-phosphorus NMR coupling constants in a series of cis- and trans-platmum(II and IV) complexes.
The trans-mftuence had hitherto been explained in terms of ATT-PTT bonding, in other words, such a mechanism dominated with trans-effect
Table 3.21 /(195Pt-15N) values (Hz) for platinum(II) amine complexes NH3 trans to
O N Cl S
Cw-Pt(NH3J2Cl2 326
fraôs-Pt(NH3)2Cl2 278 CW-Pt(NH3J2(H2O)I+ 391
frofw-Pt(NH3 )2 (H2O)2+ 312
Pt(NH3)J+ 287
Pt(NH3)3Cl+ 281 331
Pt(NH3J3(H2O)2+ 376 299
Cw-Pt(NH3)ZtIiI+ 237
C^-Pt(NH3 )2tu^+ 263
CW-Pt(NH3 )2 (SCN)2 250
Pt(NH3)3tu2+ 277 243
Pt(NH3)3SCN+ 282 264
Pt(NH3)3(Me2SO)2+ 303 243
Table 3.22 NMR and IR data [154] for complexes trans- Pt(Pbz3)2HX and mz>w-[Pt(Pbz3)2HL]+
X/L z/(Pt-H) 6 /(Pt-H) /(Pt-P) (cm"1) (ppm) (Hz) (Hz) H 1734 -2.4 800
Cl 2210 -17.36 1290 2969 Br 2221 -16.47 1345 2925 I 2192 -13.62 1359 2882 SCN 2203 -13.58 1186
CN 2059 -8.69 776
NH3 2256 -18.16 1042 2928 Py 2290 -18.89 1022 2919 PPh3 2140,2123 -6.87 783 2717 PbZ3 2145 -7.28 714 2682 P(OPh)3 2165 -5.85 758 2620 SMe2 2219 -13.30 1094 2820 CO 2207 -6.12 840
tu 2205 -10.09 1134 NO2 2200 -12.09 1008
and Zr<ms-influence. The results in Table 3.23 show that the ratio Jcis : Jtrans is similar in the platinum(II) and platinum(IV) complexes.
Since TT bonding is believed to be more important in low oxidation states, as d orbitals contract with increasing oxidation state leading to poorer d7r-p7r overlap, this would not be expected on the basis of a 7r-bonding mechanism.
Similarly, one can compare /(Pt-P) for pairs of isomers in the +2 and +4 states; in a planar platinum(II) complex, the platinum 6s orbital is shared by four ligands whereas in an octahedral platinum(IV) complex it is shared by six ligands. Therefore, the 6s character is expected to be only 2/3 as much in the platinum(IV) complexes, correlating well with the /(Pt-P) values, which can be taken to be a measure of the cr-character in the bond.
Table 3.23 NMR coupling constants for plati- num phosphine complexes [155]
/(195Pt-31P) (Hz) a5-PtCl2(PBu3)2 3508 CW-PtBr2(PBUs)2 3479 c/5-PtI2(PBu3)2 3372
^YZAW-PtCl2(PBUs)2 2380 fraôs-PtBr2(PBu3)2 2334 trans-?i\2 (PBu3 )2 2200 Cw-PtCl4(PBUs)2 2070 /JYWw-PtCl4(PBUs)2 1462 cw-PtCl2[P(OEt)3]2 5698 CW-PtBr2[P(OEt)3I2 5662 cw-PtI2[P(OEt)3]2 5472
Table 3.24 NMR coupling constants for platinum amine complexes [156]
/(195Pt-15N) (Hz) CW-PtL2Cl2 351 C^-PtL2Cl4 249 JfWi^PtL2Cl2 290 Cw-PtL2Br2 334 Cw-PtL2Br4 223 JTWw-PtL2Br2 279 L = C12H25NH2
Support for this view is found in the 195Pt-15N coupling constants for dodecylamine complexes of platinum(II) and platinum(IV), where 7r-bonding cannot of course occur, which exhibit similar trends (Table 3.24) [156].
As already mentioned, a purely 7r-bonding mechanism cannot account for the position of hydride in trans-effect and /r<ms-influence series. Overall, therefore, a major role (though not necessarily the only one) for cr-bonding is implied.