Palladium clusters, linear

Fig. 51. Scanning electron microscope image of different stages of metalization of DNA. (a) Linear chain of separated palladium clusters connecting two gold contacts (b) magnification of (a) showing clusters with diameter > 40 nm (c) continuous coated DNA strand after one development step with a diameter larger than 40 nm. Reproduced with permission from Ref. (175). Copyright 2001, American Institute of Physics.

Reaction Mechanism. To understand the size-dependent reactivity of palladium clusters on MgO surfaces in more detail, combined Fourier transform infrared (FTIR) and thermal desorption (TDS) studies were performed. The cluster model catalysts were first exposed to 1 Langmuir of CO at 90 K and subsequently to the same amount of NO. Upon linearly heating the model catalysts, the product molecules C02 and were detected by mass spectrometry as a function of the cluster size for Pd with n < 30. While for Pd4, the formation of C02 is negligible, Pdg and Pdso form C02 at 305 K or 145K and 300K, respectively (Fig. 1.97). 

We will start by presenting a case of sandwiched linear palladium clusters [53]. If a delocalized view is taken, it might not be easy to explain the reason... 

F. 16 Examples of sandwiched linear palladium clusters [S3] and their corresponding valency counting pictures [54]. Each square around Pd denotes that the center has a I64 configuration as defined in Sect. 5.3.1... 

Balch et al. have described the preparation of (18 equation 10).107 This unusual cluster contains a linear array of palladium atoms, each palladium atom having a square planar geometry and Pd—Pd 2.592 A. 

The conclusion that palladium particles in zeolites may carry a partial positive charge follows from the IR study of CO adsorption. This adsorbate can be considered to be a probe of the electronic state of palladium. Namely, the shift toward higher frequencies of the CO linear band (for Pd°-CO it appears at <2100 cm ) reflects a decrease in the back donation of electrons from Pd to CO. Along with such an interpretation, Figueras et al. (138) detected the presence of electron-deficient Pd species in Pd/ HY but not in Pd/Si02. More recently, Lokhov and Davydov (139) confirmed the presence of positively charged Pd species apart from Pd° in reduced (at 300°C) Pd/Y samples and ascribed a 2120- to 2140-cm"1 band to Pd+-CO complexes (Fig. 7). Similarly, Romannikov et al. (140) report that adsorption of CO on Pd/Y samples reduced at 300°C produces IR bands at >2100 cm 1 ascribed to Pd+-CO and Pdzeolite protons, because the IR band of the zeolite O-H group decreases when CO is released and increases when CO is added to the cluster (141, 142). 

Addition of PPhi to the linear cluster [Pd3(CNMe)8] affords the substitution of CNMe and gives the linear cluster [Pd3(CNMe)6(PPh3)2]" where the three palladium atoms are colinear with the two phosphorous atoms of triphenylphosphine groups, forming a five atoms chain. 

Elements of this group, like metals of groups 8 and 9, form mononuclear and cluster homoleptic complexes in which metals adopt a zero oxidation state. The following homoleptic complexes of these elements are known [M(CNR)4] (M = Ni, Pd, Pt), [Ni(CNR)4], [Ni4(CNR),], [Ni4(CNR)4] , [NigiCNR). ], [Ni4(CNR)i4](C104)s, and [Ni3(CNR),i](C104)6. Linear structures were assigned for the last two compounds.Monomeric platinum and palladium compounds... 

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