The Nickel Triad

The effect of a range of X groups on the UV-visible spectrum of the complexes as well as their electrochemical properties was studied when X is election donating, oxidation to Ni(ni) is made easier. 

A kinetic study of the thermolysis of trms- [Pd(Et)(X)(PMe3)2] has been carried out. Two routes yielding ethylene or ethylene plus ethane were identified resulting from loss of X or without loss of X. l23 Oxidative addition of methyl iodide or benzyl iodide to [Pd(Me)(Ph)(bipy)] produced the Pd(IV) products [Pd(Me)(R)2(X)(bipy)]. These products are capable of transferring both R and X to another Pd(II) compound at low temperature. The reaction of methyl iodide with [Pd(Me)2(bipy)] was studied as a function of pressure and concentration. The volume of activation supports an Sn2 mechanism for the forward reaction producing/rc-[Pd(Me)3(l)(bipy)]. The reverse reaction was also studied m detail and bond cleavage or partial metal reduction occurs in the transition state. 

Reaction of potassium tris(pyrazolyl)borate with [Pd(Me)2(pyridazine)] or [Pd(Me)(Ph)(tmeda)] produces the anions [Pd(Me)(R)(HB pyrazolyl 3)], R = Me, Ph respectively. Treatment with wet ketone produces the Pd(IV) alkyls [Pd(Me)2(R)(HB pyrazolyl)3)]. The trimethyl product has been ciystallographically charactmsed. The net reaction is oxidation of Pd(II) by water accompanied by a methyl group transfer. Allenyl and propargyl halides add trans across the metal centres in [Pd(PPh3)4] and [Pt(PPh3)4], The isomerisation of the o- allenyl products [M(X)(CH=C=CRR )] has been studied and the crystal structure of one example has been determined. 

A-frame cations have been produced from the reactions of [Pt(Cl)(R )(ii -COD)2] or (Pd(Me)-(AsPh3)]2(p.-Cl)2) with [Pt(R)(dppm)(Ph2CH2PPh2-P)][PF s] which contains both a bidentate and a raonodentate dppm ligand. The products, (Pt(R)](p-Cl)( i-dppm)2[M(R )] [PFg], where R, R = Me, Et, and Ph, and M = Pt, Pd, could be converted to the bridging hydrido cations by reaction with sodium borohydride. 28 An unusual redistribution reaction of [Pd(PMc3)4] and [Pd(hfac)2] in the presence of two equivalents of trimethylphosphine produced the metal-metal bonded dimeric sah, [Pd2(PMe3)6][hfac]2 which has been structurally characterised it contains two square planar metal centres with the planes mutually perpendicular. The salt was volatilised at 100 C to yield inter alia [Pd(Me)(PMe3)3][hfac] whose formation implies P-C bond scission. The mixed palladium -platinum dimer dication was also prepared. 

Oxidative addition of aryl halides to [Pd(dba)2] in the presence of tetramethylethylenediamine or 2,2 -bipyridine to produce cis- [Pd(N-N)(Ph)X] proceeds easily when iodides are used but with more difficulty with bromides. Methyllithium reacts with the iodo complexes to give the mixed dialkyls. Cross coupling of alkenylboronic and arylboronic acids with alkenes is catalysed by palladium acetate in acetic acid. The reaction is believed to occur by oxidative addition of the carbon-boron bond across an in situ Pd(0) centre. 35 

The Nickel Triad,—The tetranuclear complex [ Ni2(CO)20 -C3Cl3XiM-Cl) 2] (36), formed from nickel tetracarbonyl and tetrachlorocyclopropene, contains ring-opened trichloropropenyl ligands bridging bonded pairs of nickel atoms. The complexes [NiPh(acac)(PR3)] react with organic halides (R X) to form cross-coupled products (Ph-R ), and undergo insertion and jS-elimination reactions with olefins to yield phenyl substituted organic products. The methyl complexes 

Linear and cyclic 2-chlorovinyl ketones react with [Pd(PPhg)4] to afford new / -carbonylvinylpalIadium(ii) complexes ra/i5-[PdCl CHCHC(0)C6H4R (PPh a) ] 

The mechanism and stereochemistry of c clobutenylpalladium ring-opening reactions have been actively investigated. The first palladacyclopentane 

Onishi, H. Yamamoto, and K. Hiraki, Bull. Ghent. Soc, Japan, 1978, 51, 1856. 

Lenarda, N. Bresciani-Pahor, M. Calligaris, M. Graziani, and L. Randaccio, Inorg, Chim. Acta, 1978,26, L19. 

3 The Nickel Triad.- The chemistry of square-planar dialkyl 

A molecular orbital analysis of the addition of neutral ligands to cis-[NiR2L2] complexes, which promotes the reductive elimination of R-R, has been carried out showing how the fifth ligand affects 

The dialkyl derivatives, [NiR2(dmpe) ] (R=CH2SiMe3, CH2CMe2Ph) were obtained by phosphine replacement of pyridine or TMED in the corresponding [Ni CH2SiMe3) 2 (py) 2 [Ni (CH2CMe2Ph ) 2 (TMED )] 

The molecular structures of trans-[Ni C H3(OMe)3-2,6 2(PR3)2] (PR3=PM63,PMe2Ph) and their m-brominated derivatives trans-[Ni CgH(OMe)2 2,6-Br2 3,5 2(PR3)2 have been determined and show 

Thermolysis of trans-[Ni(CCl CCl ) (CgH,jY) (PMe3 2] (Y=NMe2 4, Me-4, Me-3, or Cl-4) yields the isomeric complexes trans-[Ni C2CI2(CgH.Y) C1(PMe,)2], the products expected from reductive 

3 The Nickel Triad. - The square-planar nickel(II) complex [Ni(Me)(PMe3)(2-phosphanylphenolato)] and the five-coordinate species [Ni(Me)(PMe3)2(2-phosphanylphenolato)] have been prepared and found to be effective one-component catalysts for the oligomerisation of ethene. The aryl-nickel(II) complexes 38 have been found to be remarkably efficient catalysts for 

Solution calorimetry has been used to measure the reaction enthalpies of the complex [Pt(Me)2(Ti -C8Hi2)] with a range of bidentate phosphines and it has 

The rates of chloride dissociation for triphenylphosphine substitution have been measured for a series of cyclometalated [Pt(N-N-C)Cl] complexes containing terdentate N-N-C anionic ligands derived from deprotonated alkyl-, phenyl-and benzyl-6-substituted 2,2 -bipyridines. The main factor controlling the rate is 

C(R)CH = C(R)C(H)PPh3 ][PF6]2. The complexes [Pd2(dba)3].CHCl3 and [Pd(PPh3)4] both react with the phosphino ylide [Ph2PCH2- 

Asymmetric dimerisation of 3,4-dimethyl-1-phenylphosphole in the presence of an organoplatinum(II) complex derived from (R)-Ar,AT-dimethyl-l-(l-naph-thyl)ethylamine affords an optically pure P-chiral diphosphine. Reaction of racemic Sb-chiral (+ )-l-phenyl-2-trimethylsilylstibindole with di-p-chloro-bis (iS)-2-[l-(dimethylamino)ethyl]phenyl-C,Ar dipalladium(II) gives a 1 1 mixture of the diastereomeric complexes 51a and 51b. Separation of the complexes by chromatography followed by treatment with triphenylphosphine affords the optically pure l-phenyl-2-trimethylsilylstibinoles in quantitative yields. The 

An ab initio study of the oxidative addition of CH4 to the Ni(PH3)2 fragment showed that the reaction is endothomic with the trans product favoured.358 Calculations predicting that ketenes would adopt the ll2-(C,0) bonding mode were borne out by the synthesis of 

The preparations of [NiR2(NCPh)2] (R = CgFs), [NBu4]2[NiR2di-OH)]2, and [NBu4][NiR2L] 

The dissociative chemisrapdon of MeOH on a Pd(l 11) surface to give methyl groups was found to be coverage-dependent. Decomposition of diazirine on a Pd(110) surface proceeds by cleavage of C-N and N-N bonds.38l The oxidative addition of CH4 and C2H5 to Pd and Pt has been examined by theoretical methods.268.382.383 Various catalytically important Pd(0) and Pd(II) cmnplexes have been studied by ESCA photoelectron specoosct y.  

A short review also mentions related leactions. The complex rrflns-[PdCl(COPh)(PPh3)2] has been used as an aryladon catalyst  

The insertion of DMAD into the phosphinoenolate complex [(dmba)F d Ph2PCHgC(OEt)(j )] (dmbaH = HiQrl2NMe2) produces the metallacycle [(dmba)P d Ki2PCH(C O2E0CR=CiR) ] (15) (R = C02Me) insertion of TCNE gives a related product. The X-ray structure of a Pd2 A- ame complex with a p, i), tiI-DMAD ligand has been reported.  

LiMe moiety forms a (r -bond to nickel in complexes of general 

TT-ligand=l,5,9-cdt, ethene,or CO), as shown by the crystal structure of the [Ni(LiMe ) (PMDT)(C2H )23 (17) derivative. 

The reactions of a number of substituted allenes with [Ni(PPh3)3] have been investigated and the factors affecting the regio- and stereochemistry of iT-complex formation and coupling to form nickelacyclopentane complexes have been analysed. 

An initio M.O. study has determined the optimised transition state structure for the carbonyl insertion reaction of 

The oxidative addition of a number of alkyl and aryl halides 

Formal insertion of oxygen into the Ni-C bond in [(bipy)Ni(CH2)4] is achieved by treatment with N2O to give [(bipy)Ni(CH2CH2CH2CH20)l similarly [(bipy)NiEt2] yields [(bipy)NiEt(OEt)].357 Oxidative addition of cyclic anhydrides such as phthalic anhydride to 

The reaction of [(tmeda)PdMe2] with 9S3 (1,4,7-trithiacyclononane) affords [(9S3)PdMe2l related Pd and Pt complexes with other alkyl groups were also made. Variable temperature NMR studies suggest exchange between coordinated and uncoordinated sulfur atoms. Addition of Mel produces Pd(IV) complexes such as [(9S3)PdMe3] T which shows no tendency to eliminate C2H6.379 Related Pd(II) and Pd(IV) alkyls have been studied by electro ray mass spectrometry, where facile elimination was observed.380 

6- C6H3pr 2 or CHMePh R = C02Me) with alkenes or alkynes affords 1 1,2 1 and 2 2 addition products.397 The cyclisation of enynes with palladacyclopentadiene catalysts has continued to be exploited.398-400 xhe alkynyl complex [(dppe)MMe(OCPh)] has been made from [(dppe)PdMe OCH(CF3)2 ] and PhC=CH thermolysis releases MeOCPh. 0i 

Reaction of LiR (R = CH2S02tol) with [Pd2Br6] afforded [Pd2(li-Br)2R4] which could be converted into [PdR2(NCMe)2] and thence to a variety of other derivatives. 12 

A comparison of the activity of [Pd(PPh3)4] and [Pd(dba)2] + PPhs in the oxidative addition of Phi concluded that for an equivalent Pd loading the former contained more of the active species [Pd(PPh3 2]. 5 An electrochemical and 31p NMR study of the mechanism of addition of PhX to reduced [PdCl2(PPh3)2] has also appeared. e New aryl complexes have been formed by addition of rrans-4 -bromo-4-stilbazoles and 2,4,6-tribromophosphorin to [PdLa] (L = various 

Heating [Ir(acac)3] with 2-phenylpyridine (Hppy) affords the tris-cyclometallated complex [Ir(ppy)3],392 and the synthesis of other complexes of the ppy ligand have also been reported.393,394 cyclometallated benzoquinoline Ir complex was shown to be an active catalyst for the H/D exchange of alcohols with D2.395 2.3 The Nickel Triad 

The bonding of Me to a Ni(l 11) surface and the energy of its formation from adsorbed CH2 and H have been investigated by theoretical means.396 The rates of reaction of alkyl radicals with [Ni(cyclam)]2+ to form [RNi(cyclam)(H20)]7+ have been studied, showing that Me primary secondary.397.398 The preparations of (Ni(NSR3)R ] [NSR3 = N(CH2CH2SR)3 where R = Pr, Bu R = Me, Et, COMe) have been detailed.399,400 

C6H402)(PMe3)3] was fonne(i. 04 protonation of [L2M(ti-C2H4)] (M = Ni, Pd L2 = bulky chelating phosphine such as Bu PCH2PBu4) gave agostic ethyl complexes. OS 

Dissociative chemisorption of Mel onto a Pd(lOO) surface gives surface Me groups of less stability then those formed from MeOH on Pd(l 11). 33 a theoretical study of the insertion of CO into the Pd-H or Pd-Me bonds of [PdH(R)(CO)(PH3)] (R = H, Me) concluded that the former was more unfavourable because of the Pd-H bond strength.434 Calculations showed that a migration mechanism for reductive elimination in [PdMe(CH=CH2)(PH3)2] was favoured if the P-Pd-P angle was flexible. 35 

Exchanging the halogen in [PdMe2X(CH2Ph)(bipy)] affects the ratio of C2H6 and ethylbenzene formed on reductive elimination.436 The first Pd(IV) aryl complexes [PdMePh(X)(R)(bipy)J (R = Me, X I R = CH2Ph, X = Br) have been made by addition of RX to [PdMePh(bipy)] reductive elimination occurs above 0°C to form mostly ethane for R = Me and toluene for R = CH2Ph.  

Stobbe, G. Wilke, R. Benn, R. Mynott, K. Seevogel, 

Schiavon, G. Bontempelli, M. De Nobili, and B. Corain, Inorg. Chim. Acta, 1980, 42, 211. 

Coronas, C. Polo, and J. Sales, Synth. React. Inorg. Metal-Org. Chem., 1980, 10, 53. 9 R. Uson, J. Forni6s, R. Navarro, and A. Gallo, Transition Met. Chem., 1980, 5, 284. 

Nakatsu, K. Kinoshita, H. Kanda, K. Isobe, Y. Nakamura, and S. Kawaguchi, Chem. Lett., 1980, 913. 

Diversi, G. Ingrosso, G. Lucherini, and S. Murtas, J. Chem. See., Dalton Trans., 1980, 1633. 

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The optical spectra of the nickel-triad metals have been reinvestigated (111) in Ar, Kr, and Xe matrices, and, although the data for Ni and Pt atoms correlated well both with previous studies and with the... 

The metal-atom reactions of Cu (98), Ag (134), and Au (135) with O provided interesting results, especially when these were compared with the results from the nickel triad (137). As shown by standard matrix-techniques, Ag forms two O2 complexes that are best formulated as Ag 02 and Ag+Oj, based on the absence of visible absorptions and the similarity of the IR spectra to those of Cs" 02 and Cs Oj (3a,b). The UV absorptions for Ag(02) and AgiOJ, at 275 and 290 nm, respectively, could be associated with the O2 and O4 anions. The shifts in the IR spectra on going from Ag(02) to Ag(04) also argue against an (02)Ag(02) formulation for the latter complex, being in the opposite sense to those observed for Pd(02)ian[Pg.139]

The chemistry of complexes of members of the nickel triad with 1,1-ligands has been studied extensively (1). 

A variety of behaviour is observed in the reactions of [Na(THF)4][cyclo-P5 Bu4] with phosphine halide complexes of the nickel triad." Of particular interest is the formation of the complex Ni(cyclo-P5 Bu3)(PEt3)2 in which the... 

Among the characterized metal homoenolates, only zinc homoenolate of alkyl propionate undergoes substitution reactions with electrophiles under suitable conditions. Two types of metal catalysts, copper(I) and metals of the nickel triad (e.g. Pd), have successfully been used to effect allylation, arylation, and vinylation reactions. 

Of the transition metals, the nickel triad has been the most studied in terms of xanthate structures, especially nickel xanthates. Thus, binary xanthate structures are known for all elements with no less than 15 binary nickel xanthate structures known and these aggregate in the solid state in six distinct supramolecular architectures. There is also a rich diversity of adducts as well as mixed-ligand complexes, in particular for nickel. 

The carbonylation of alcohols can proceed with formation of carboxylic acid by catalytic insertion of CO into the carbon-oxygen bond. An alternative reaction gives rise to oxalate or formate esters, when the CO is inserted into the oxygen-hydrogen bond. The members of the nickel triad carbonylate alcohols to give each of these products, and they will be discussed separately. 

An interesting addition reaction has been found with the isocyanide complexes of the nickel triad and dialkylamines [Eq. (142)]. The metal atom is oxidized, with formation of diaminocarbenes (77). The ligand cis to the metal-bonded oxygen is attacked. This is shown in the X-ray structure of 177b (197). 

These conditions are typically fulfilled by transition metals with d8 and dw electron configurations, notably Fe°, Ru°, Os°, Rh1, Ir1, Ni°, Pd°, Ptn, and Pt° (see later for a discussion of Niu/NiIV and Pdn/PdIV). The stability of the higher oxidation states increases within a transition metal triad, so that, for example, Ir111 is more stable than Rh111. This trend is most pronounced for the nickel triad where there are numerous examples of the oxidation of Pt11 to Ptlv but stable products of oxidative addition to Pd11 have only recently been discovered. 

The N,0,P phosphine-imine-alcohol donor (122) forms simple square-planar complexes with M ions of the nickel triad. The analogous ether (123) acts as a terdentate ligand to Pd, with CH3 in the fourth site of the square-planar complex. ... 

In the next sections, the synthesis, reactions, and relevance to catalysis of hydro complexes of the nickel triad are discussed. A short section on spectroscopy of these compounds is included, along with an extensive table of hydro complexes that have been characterized or isolated. 

In Section II,C, the oxidative-addition reaction of protonic acids to low-valent complexes was discussed. This reaction can lead to formation of either the hydro complex or to the bis complex of the conjugate base. Both of these reactions have been reported for the addition of silanes to low-valent complexes of the nickel triad (73, 74). It is to be expected that the nature of the product will depend on the strength of the acid HX and the trans influence of the conjugate base X. If the acid is strong and the conjugate base has a large trans influence, then it would be anticipated... 

Addition of active methylene compounds to butadiene has been catalyzed by complexes of the nickel triad. A catalyst mixture of Ni(acac)2, PPr(OR )2, and borohydride was effective for the addition of R1R2CH2 (Ri = Ph, R2 = COCH3 Ri = Ph, R2 = CN Ri = Ra = COgEt Ri = COCH3, Rg = COgEt) to butadiene 26). Both Pd(0) and Pd(II) complexes are effective as catalysts for the addition of active methylene compounds such as 2,4-pentanedione and ethyl acetoacetate to butadiene 177, 269). Several products were obtained from these reactions since both the 1 1 and 1 2 addition occurred. 

M. Kumada Some reactions involving silicon and metal complexes of the nickel triad. 

Halogen oxidation of planar complexes, particularly of the nickel triads that contain bidentate glyoximes or macrocyclic ligands, such as, phthalocyanines, substituted porphyrins, or tetraazaannulenes, leads to highly conducting substances that may possess chains of metal atoms as well as chains of polyhalides ... 

Copper(I) and metals of the nickel triad have been used to effect allylation, aryladon, vinylation and acylation reactions of zinc homoenolates. Transient homoenolates generated under Pd catalysis and Ag catalysis have also been used for direct arylation and acylation of silyloxycyclopropanes. 

Collaborative Research with Chatt on the Kinetic Trans Effect of the Nickel Triad Metal Complexes... 

By the late 1950s, Chatt and Bernard Shaw had succeeded in preparing several alkyl-, aryl- and hydrido-metal complexes of the nickel triad. Because of our mutual interest in the kinetic trans effect of platinum(ii) complexes, Chatt and I decided to examine the rates of ligand substitution of these new organometallics. Shaw prepared the compounds and Harry Gray did the kinetic studies. Years later Chatt gave the following account of why the research was so rapidly accomplished. 

The stable cobalt complex LI is prepared by cocondensing cobalt atoms and butadiene at liquid nitrogen temperature This technique is also used to prepare 7t-allyl complexes of the nickel triad metals, e.g., LII . Metal hydrides are intermediates in these reactions , as shown by the reaction of nickel hydrides with dienes to yield rt-allyl complexes . The 7i-allyl complex LIII may be prepared by adding HCl and butadiene to (PPh3)4Ni in toluene or THF at —78°C, then warming to RT . ... 

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