Page 320 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: GPA Final Pages
Encyclopedia of Physical Science and Technology EN010b-481 July 14, 2001 18:45

474 Noble Metals (Chemistry)

The organometallic chemistry of platinum(IV) is lim- or alcohol with palladium(0). More π-allyl complexes are
ited to the alkyl derivatives. They are formed either by known with palladium than with any other metal.
6
oxidative addition starting with platinum(II) or by dis- Palladium(IV), d , is the second most common oxida-
placement using a Grignard reagent. tion state for palladium, but it is not as common as the
The formal platinum(−I) state is present in cluster com- platinum(IV) state. The salts include mainly the fluoride,
pounds such as [Pt 3 (CO) 3 ] 2− and other Pt–Pt bonded oxide, and nitrate counterions. Six-coordinate complexes
7
species such as [C 5 H 5 Pt(CO)] 2 . Platinum(III), d , has are known involving fluoro, chloro, phosphine, arsine, and
been proposed in PtX 3 , X = Cl, Br, I, and Pt(NH 3 ) 2 Br 3 , amine ligands, but most compounds are only stable in the
but the compounds are mixed valence species of platinum presence of excess oxidizing agent.
5
9
(II) and platinum(IV). Platinum(V), d , and platinum(VI), Palladium(I), d , is found in a few complexes of the type
7
4
d , are represented by fluoro and oxyfluoro derivatives [PdCl(CO)] n and PdCl(CO)PPh 3 . Palladium(III), d ,is
[e.g., PtF 5 , PtOF 3 for platinum(V) and PtF 6 , PtOF 4 , and formally present in “PdF 3 ” and “Pd(NH 3 ) 2 Cl 3 ”. However,
PtO 3 for platinum(VI)]. These compounds are strong ox- the paramagnetism of “PdF 3 ” has been shown to be caused
IV
II
idizing agents. by high-spin palladium(II) in Pd [Pd F 6 ] and not by the
7
d configuration of palladium(III).
4. Palladium
5. Iridium
Palladium is found in the (0) through (IV) oxidation
states. The four-coordinate square planar complexes of Iridium compounds are known for oxidation states (−II)–
palladium(II) are the most common. Figure 4 outlines (VI); the (I), (III), and (V) states are the most common.
some common palladium chemistry. The metal is easily The metal itself is very noble and will not dissolve in
oxidized. It dissolves in aqua regia and in HCl or HNO 3 aqua regia, but it will react with oxygen or chlorine at red
in the presence of oxygen to form H 2 [PdCl 4 ]. heat to give the respective salts, and it can be converted
The palladium(0) complexes [e.g., Pd(PPh 3 ) 4 and to IrO 2 by peroxide fusion. The oxide, IrO 2 , can also be
[Pd(dba) 2 (dba = dibenzylidene-acetone)] are formed by obtained by reaction of the metal with alcohol and KNO 3 .
reduction of the palladium(II) analog. Alkyne, alkene, It is used for catalysts and for electrodes. The chloride is
and nitroso derivatives are also known, the more stable formed by reaction with chlorine and KCl or HCl and Na-
palladium(0) complexes being formed with ligands hav- ClO 4 . Iridium(0) complexes are derived from iridium(I)
ing strong π-acceptor ability. and iridium(III) compounds by reduction. The [Ir(CO) 4 ] n
8
Palladium(II),d ,saltsincludethehalide,oxide,hydox- and Ir 4 (CO) 12 cluster species are standard examples.
ide, sulfide, selenide, telluride, nitrate, sulfate, cyanide, In the absence of auxiliary ligands, the iridium(I),
8
and thiocyanide derivatives. PdCl 2 is a coordination d , salts are limited to the chloride, bromide, and io-
polymer, insoluble in water, but soluble in aqueous chlo- dide, which are obtained by decomposition of the irid-
ride solution. It is used in catalysis and for galvaniza- ium(III) analogs. The major chemistry of this oxidation
tion. Stable complexes are formed with halide, nitrogen, state involves complexes and organometallics. Iridium(I)
phosphorus, arsenic, and antimony donor atoms. complexes, in general, incorporate π-acceptor ligands to
[Pd(NH 3 ) 2 Cl 2 ], [Pd(NH 3 ) 4 ]Cl 2 , [Pd(NH 3 ) 2 (NO 3 ) 2 ], and stabilize the low oxidation state, and they undergo oxida-
[Pd(CH 3 COO) 2 ] 3 are important commercial products for tive additions easily to form iridium(III) complexes. The
the electronics industry. Oxygen donor and fluoro com- species commonly have carbonyl, nitrosyl, phosphine, ar-
plexes do not form as readily. An exception to this is the sine, olefin, diene, and acetylene ligands. Two of the most
highstabilityoftheacetylacetonatecomplex.Palladium— important iridium(I) complexes are Vaska’s compound,
like platinum—forms ligand bridged complexes. In four- IrCl(CO)(PPh 3 ) 2 , which is a good model for the activa-
coordinate Pd(II) complexes the fifth and sixth coordina- tion and oxidative addition of simple molecules (e.g., hy-
tion sites are taken up by weakly bound solvent molecules. drogen, chlorine, oxygen, and HCl), and [Ir(COD)Cl] 2
As with platinum(II), palladium(II) complexes are well (COD = cyclooctadiene-1,5), which is a catalyst.
6
suited for structural and mechanistic studies. Simple iridium(III), d , salts may contain halide, oxide,
The organometallic chemistry of palladium(II) is simi- hydroxide, sulfide, and selenide ligands. IrCl 3 · (H 2 O) n is
lar to that of platinum(II) except that the palladium com- solubleinwaterandisthebasisforthepreparationofmany
pounds are less stable. This lability permits a wide variety other products (catalysts, electrodes etc.). The complexes
of useful catalytic reactions (e.g., palladium olefin com- are usually six-coordinate and rather inert. A large num-
plexes in the Wacker process). Prominent examples are the ber of complexes are known, incorporating ligands with
formation and reaction of π-allyl complexes. The π-allyl donor atoms such as oxygen, nitrogen, phosphorus, ar-
complexes can be formed from an olefin bound to palla- senic, and antimony. Iridium(III) forms a particularly large
dium(II) on heating or by the reaction of an allyl halide number of hydride complexes, mainly with phosphine and