Complexes of Phosphines and Arsines

Ligands such as phosphines (PRj) and arsines (AsRs) (R = alkyl, aryl, halogen, etc.) form complexes with a variety of metals in various oxidation states. Vibrational spectroscopy has been used extensively to determine the structures of these compounds and to discuss the nature of the metal-phosphorus (M-P) bonding. Verkade reviewed spectroscopic studies of M-P bonding with emphasis on cyclic phosphine ligands. 

The most simple phosphine ligand is PHj. The vibrational spectra of Ni(PH3)4, Ni(PH,)(CO)3, and Ni(PHj)(PF3)3 have been reported by Bigorgne and co-workers. All these compounds exhibit r(PH), S(PH3), and (NiP) at 2370-2300, 1120-1000, and 340-295 cm , respectively. A series of the Ni(PHj) - (n = I 4) type complexes have been prepared by matrix cocondensation reactions, and their r(Ni-P) assigned at 390-350 cm . Complete assignments based on normal coordinate calculations have been made on Ni(P(CH3)3)4. The A, and Fy v(Ni-P) vibrations of this compound have been assigned at 296 and 343 cm , respectively. 

Tabu- lH-63. Infrared Frequencies, Isotopic Shifts, and Band Assignments of NiXaCPEtjlj (X = C1 and Br) 

PEtj V NiCl2(PEtj)2 NIBr2(PEt3)2 Assignment  

As stated in Sec. II -19, complexes of the type Ni(PPh7R)2Br2 (R = alkyl) exist in two forms (tetrahedral and square-planar) which can be distinguished by the p(NiBr) and p(NiP). For R= Et, the (NiP) of the planar complex is at 243cm , whereas these vibrations are at 195 and 182 cm in the tetrahedral complex. 

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Platinum(IV) complexes of phosphine and arsine ligands formed by oxidation with chlorine or bromine are well known. Chloro complexes such as ci5-[PtCl2(PEt3)4]2+ can be generated by electrolysis of the platinum(II) complex c -[PtCl2(PEt3)2].433 Similar platinum(IV) complexes... 

The coordination of the nitrate groups in the complexes of phosphine and arsine oxides already discussed is generally symmetrically bidentate. However, in Rb2[Sc(N03)5] there are three biden-tate and two monodentate nitrates, resulting in eight-coordination for scandium, in contrast to (N0" ")2[Sc(N03)5] , where one monodentate and four bidentate nitrates give nine-coordinate scandium. ... 

Selected nickel(I) complexes with phosphines and arsines, together with relevant properties and synthetic routes, are reported in Table 21. In general, these nickel(I) complexes are air-unstable, especially when dissolved in solution consequently their preparation and handling require the exclusion of oxygen and, often, of moisture. The synthetic routes which afford nickel(I) complexes are strictly dependent on the nature of the phosphines and arsines and are not of general application, except in the case of some tripodal ligands. Most of the nickel(I)... 

Table 21 Summary of Synthetic Procedures and Properties of Nickel(I) Complexes with Phosphines and Arsines... 

Numerous nickel(II) complexes with a variety of phosphine and arsine oxides have been reported, but only a few X-ray crystal structures have been determined. In some cases the structures assigned to the complexes are not completely certain.1800 A selection of nickel(II) complexes is reported in Table 86. 

A number of nickel(III) complexes with phosphines and arsines have been isolated, but nickel(IV) complexes with the same ligands are still rare, o-phenylenebisdimethylarsine (diars) and o-phenylenebisdimethylphosphine (diphos) have been found very efficient in stabilizing nickel(IV). Selected examples of nickel(III) and nickel(IV) complexes with phosphines and arsines are shown in Table 119. 

Some interest in group 12 phosphide and arsenide complexes has come from their possible applications as precursors for IFV semiconductors (see Semiconductors). They can be prepared by two different ways, namely, by metaUa-tion reaction of phosphines and arsines with diorganozinc compounds (equation 32), and by transmetallation reaction between organozinc halides and lithium salt of the corresponding phosphines and arsines (equation 33). 

The induction period can be explained as the result of reactions a or /3. The first, proposed by Heck 89), requires that tetracarbonylnickel have basic properties, but this could not be proved. In fact, the protonation reaction of carbonyls and carbonyl derivatives has been studied by Wilkinson and co-workers 64) Ni(CO)4 does react with acids but, contrary to Fe(CO)5, gives no NMR signal—indicating the formation of a hydride. However, the basic properties of some systems have been recently shown in complexes with phosphines and arsines. The platinum (0) 39, 40) derivatives yield stable hydrides according to the following equilibria ... 

The essentially ciass-b character of Pd" and Pt" is further indicated by the ready formation of complexes with phosphines and arsines. [M(PR3)2Xi] and the arsine analogues are... 

G. Booth, Adv. Inorg. Radiochem., 1964, 6, 1, provides a detailed early review of phosphine and arsine complexes. 

The mechanisms of the oxidation of phosphines and arsines by chromium(VI) have been examined both in solution and on a diatomite support. Kinetic parameters are presented for both supported and solution reactions. A ruthenium complex of 1,4,8,1 l-tetramethyl-l,4,8,ll-tetraazacyclotetradecane has been utilized to oxidize triphenylphosphine in acetonitrile. Although a limited temperature range was utilized, a AH value of 8.7 0.8 kcal mor and a A5 value of -20 2 cal K mor were calculated. The secondary phosphine oxides, HP(0)R (R = n-butyl, isobutyl, cyclohexyl) and 9H-9-phosphabicyclononane-9-oxide, react with cobaltocene to yield dihydrogen and cobalt(I) compounds. With the less bulky phosphorus ligands at elevated temperatures trinuclear cobalt(III, II) complexes may be obtained. Arsenious acid may be utilized to catalyze the oxygen atom... 

Measurements of gas phase dissociation constants have shown that the Lewis basicity towards hard reference acids such as BMc3, BF3 or BCI3 falls down Group V in the order MejN > Me P > MejAs > Me3Sb ( MejBi). Towards the softer acid BH3, however, the order is Me3 > MejN > MejxAs. Orders of basicity are strongly influenced by the nature of the acceptor. This is especially evident from the chemistry of transition elements, which often form stable complexes with phosphines and arsines for which there are no amine analogues (Chapter 5). Phosphines and arsines are, in the classification of Pearson, typical soft bases which form their most stable complexes with soft acids such as polarizable transition metal and heavy post transition metal acceptors. 

The oxidation of phosphine and arsine by metal complexes has been reviewed. [Cr03F] oxidizes H3PO2, and H3PO3 by inner-sphere mechanisms involving the formation of well-defined metal-substrate complexes. The first-order rate constants for decay of this intermediate are 1.31 X 10 s and 1.74 x 10 s , respectively, and the rate-limiting step is transfer of a hydride ion to the oxidant. 

The osmium(IV) complexes are only obtained by this route with fairly unreactive phosphines and arsines (e.g. PBu2Ph) but they are conveniently made by oxidation of mer-OsX3(QR3)3 (Q = P, As) with the halogen in CHC13, or CCI4 and refluxing. 

Rhodium(III) forms a wide range of complexes with tertiary phosphines and arsines [108, 109], though in some cases other oxidation states are possible. Table 2.5 summarizes the complexes produced from reaction of RhCl3 with stoichiometric quantities of the phosphine. 

A considerable number of the tertiary phosphine and arsine complexes of iridium(III) have been synthesized [4, 8] they generally contain 6-coordinate iridium and are conventionally prepared by refluxing Na2IrCl6 with the phosphine in ethanol or 2-methoxyethanol [154]... 

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