Trifluorophosphine bonding

The comparatively simple method of preparation of tetrakis (trifluorophos-phine)nickel-(0) encouraged some scouting experiments on its still unexplored chemistry. Whereas the compound is hydrolytically remarkably stable, it was found to react readily with amines and ammonia with complete aminolysis of the phosphorus-fluorine bonds. Very typical of tetrakis(trifluorophosphine) nickel-(0) and similar fluorophosphine and chlorophosphine complexes of zerovalent nickel is the rapid decomposition with precipitation of elemental nickel by aqueous alkali hydroxide. 

It was decided to study the system tetrakis (trifluorophosphine) nickel- (0) -ammonia (23) in some detail a smooth reaction was observed when the complex, condensed on excess ammonia at liquid air temperature, was allowed to warm up gradually. Precipitation of colorless crystals, identified as ammonium fluoride in almost stoichiometric amount, based on complete ammonolysis of the phosphorus-fluorine bonds, was observed at temperatures as low as —90° to —80°. Removal of the ammonium fluoride by filtration at temperatures not higher than —50°, and subsequent slow evaporation of the ammonia from the filtrate invariably led to a brown-yellow solid, although a colorless, crystalline material was formed initially. The product was decomposed almost instantaneously by water with precipitation of elemental nickel. Analysis of the hydrolyzate obtained in aqueous hydrochloric acid revealed a nickel-phosphorus-nitrogen atom ratio close to 1 4 4, corresponding to an apparently polymeric condensation product. 

There are many similarities in the behavior of PF3 and CO as ligands. This is manifested by the existence of a large number of PF3 complexes of the transition metals (197, 226). Several UPS studies of such complexes have now appeared in the literature (20, 152, 169, 182, 221, 227). As pointed out in Section II, it is difficult to arrive at a completely unequivocal set of assignments for these compounds other than for the predominantly metal MOs. It is seen below, however, that one of the advantages of studying the UPS of the PF3 complexes is that many more ionizations pertinent to bonding discussions are discernible. Furthermore, trifluorophosphine complexes tend to be more thermally stable than their carbonyl counterparts. 

A useful general synthetic approach involves the insertion of allene or dienes into the metal-hydrogen bond of transition metal hydrido-trifluorophosphine complexes (method C). 

Trifluorophosphine and carbon monoxide readily undergo ligand-exchange reactions in their transition metal complexes. The close similarity in bonding characteristics of the two ligands toward transition metals has been discussed extensively in several review articles (72,174,272) and the evidence will not be repeated here. Extensive vibrational spectroscopic studies have been made on mixed carbonyl-PF3 metal complexes (72,174) and force constant calculations have been carried out in some cases. 

Since the P—F bonds are labile in these complexes some mixed trifluorophosphine-fluorophosphite nitrosyl complexes have been prepared by reactions with either diisopropyl-water mixtures or barium hydroxide (method F) to give the (PF20) compounds which can be alkylated. 

Relatively few examples of metal trifluorophosphine complexes of formula [MRx(PF3)y are known (see Table XVI). 

Transition Metal Trifluorophosphine Complexes Containing ct-Bonded Group IV Elements... 

Alternatively, treatment of trifluorophosphine metallates with R3MC1 (method B) or via metal-metal bond cleavage reactions of dinuclear PF3 metal complexes with a group IV metal hydride R3MH (method C) have been useful synthetic methods, e.g.,... 

An important paper describes the formation of the metal-metal bonded dirhodium tetracarboxylate trifluorophosphine complex [Rh2(OCOCH3)4(PF3)2] made directly from the dirhodium tetraacetate complex by direct addition of PF3 to the formally metal-metal triple bond. The structure was determined by a single-crystal X-ray study, and has been compared with other [Rh2(OAc)4X2] systems (X = py, Et2NH, CO, and P(OR)3). 

The parent TMM complex (190 R = H) undergoes photochemical ligand substitution with trifluorophosphine or trimethylamine Al-oxide assisted substitution with tertiary phosphines or t-butyl isocyanide (Scheme 5A) Trimethylamine A-oxide assisted substitution using isoprene as the incoming ligand results in C-C bond formation to afford the bis-TT-allyl complex (197). An intramolecular version of this reaction is also known.The parent complex (190 R = H) reacts with electrophiles. Addition of HCl or Br2 gives the methallyl complexes (192) and (198), respectively. Tetrafluoroethylene adds across the Fe bond to afford (199) under photochemical conditions. Complex (190) undergoes Friedel-Crafts-type acylation with... 

Although the coordinating character of trifluorophosphine and trialkyl phosphite can be assumed to be very similar, the observed coupling constants J( Co- P) differ substantially from one another. The smaller value found in the octahedral complex has therefore been accounted for by the larger fractional s-character of the Co-P bond in the trimethyl phosphite complex. (238)... 

The positive value found for PCI3 (and for other haldgenophosphines) can be understood as an inductive effect or as a very strong t bond which deshields the phosphorus atom. Many authors prefer the first explanation. In fact, in the case of PF3 the Ni-P force constant was 2.7 (206) or 2.37 mdyne/A. (123)—a rather low value which does not indicate the presence of a double bond. On the other hand, the examination of the force constants in the series Ni(CO)4 n(PF3)n (n = 0, 1, 2, 3, and 4) indicates that PF3 has about the same properties as CO (59, 123). This is supported also by the statistical distribution of the products in the exchange reaction between Ni(CO)4 and PF3 and by the little variation of NMR chemical shift on these products (56). Loutellier and Bigorgne (123) prefer to consider trifluorophosphine as a weak a donor and weak 7T acceptor for the following reasons. 

The molecular structure of trifluorophosphine has been the subject of several papers and considerable differences in bond length and bond angles have been reported. The very early work by Brockway et al. 33, 258) gave r(P-F) = 1.52 A, 0(FPF) = 104°, and more recently Hersh (13i) found r(P-F) = 1.537 0.04 A, 0(FPF) = 98.2° 0.6°. In neither case, however, were these data consistent with microwave studies [113, 208, 209). 

Force constant calculations for Ni(PF3)4 (190, 326) indicate that K(P-F) is similar to that in free trifluorophosphine and the metal-phosphorus force constant (about 2.4 mdyne/A), is only in the range expected for a single bond. Very recently, structural data for Ni(PF3)4 have become available from electron diffraction studies (la, 279a) (Section IX,C) and the phosphorus-fluorine bond length (1.561 ... 

The similarity in the coordinating ability of CO and trifluorophosphine toward transition metals is illustrated by mass spectroscopic studies on HCo(PF3)j.(CO)4, c, which show that the Co-CO and C0-PF3 bond energies are both 56 15 kcal (279). Along the series a = 4 0, the AH formation of the complex becomes progressively more negative by about 200 kcal/mole, which is approximately the difference between... 

There is an interesting report of carbonyl substitution in the reaction between phosphorus pentafluoride and MeSiH2Co(CO)4 (114, 7). There is evidence that PF5 is reduced by the silicon-hydrogen bonds yielding HCo(CO)4, fluorosilane, and trifluorophosphine the latter subsequently displaces carbon monoxide from the hydridocarbonyl complex. 

It is noteworthy that the mixed trifluorophosphine-difluorophos-phite complexes are more thermally stable than the parent trifluorophosphine complexes. The phosphorus-fluorine bond is apparently not susceptible to further attack by Ba(0H)2. 

FsP BHg is a spontaneously inflammable gas which is appreciably dissociated at room temperature. Trifluorophosphine is readily displaced by trimethylamine 255), and treatment with ammonia between —111 °C and room temperature leads to complete cleavage of phosphorus-fluorine bonds although at —78°C there is evidence for formation of (NH2)2PF.BH3-... 

Trifluorophosphine has the highest field chemical shift (Sp) of all the phosphorus trihalides or fluorophosphines R PF3 1 3) and the unexpected order for Sp in the PX3 series (X = halogen) was originally discussed (122) in terms of opposing effects of ionic and double character of the P-X bond. A relationship apparently exists between Sp and the electronegativity of the substituent atom for some RPF2 compounds (R = F, RO-, R2N-), the values being considerably lower for the R2N derivatives than the RO compounds. 

The CoNMR spectra of the tetrahedral [Co(PF3)4] and [Co(CO)4] anions have been recorded (191) and values obtained for the V( Co- P) (1222 25 Hz) and V( Co- C) (287 13 Hz) coupling constants. Approximate values for the cr-bond order were calculated and together with shift data suggested that bonding to the ligand tt orbitals is more important in the carbonylate ion than bonding to the phosphorus 3d orbitals of the tetrakis(trifluorophosphine)cobaltate, and that the cobalt-phosphorus u-bond order apparently is greater than the cobalt-carbon c7-bond order (191). 

When manganese vapour and a mixture of nitric oxide, trifluorophosphine and boron trifluoride are co-condensed the compound Mn(PF3)(NO)3 is produced [280]. Co-condensation of nickel and carbon dioxide results in the formation of some nickel tetracarbonyl [280]. Burdett and Turner [298] showed that co-condensation of nickel and nitrogen at 20°K resulted in a nickel—nitrogen complex. Moskovits and Ozin [299] have recently repeated the experiment and have shown from the infrared spectrum of the matrix that the major product is NiN2, with the nickel atom bonded to the end of the nitrogen molecule. 

Particularly stable complexes can be formed with PF3. Although the highly electronegadve F atoms significantly lower the ability of the phosphorus atom to donate its lone-pair electrons in o bond formation this is compensated by the Ji bonding capacity of the vacant d orbitals. Trifluorophosphine forms complexes analogous to those of carbon monoxide, for example, F3P BH3 and Ni(PF3)4 which can be compared with the very stable H3B CO and Ni(CO)4. A whole series of complexes of general formulae [M(PF3)where jc = 1 or 2, are known, which correspond to carbonyl complexes with similar formulae (8.173). 

Receiving two electrons from each of four PF3 groups, nickel acquires the noble-gas electron configuration in tetrakis(trifluorophosphine)nickel, Ni(PF3>4. Two independent studies of this molecule differ in their interpretation of the internal motion. One led to the conclusion that the non-bonded P P distance is ill-defined,although it does indicate tetrahedral co-ordination around nickel a large amplitude of vibration for P---P was obtained. The preferred molecule conformation is one which has each PF3 group twisted 40° from the position where it eclipses the NiPi skeleton. This model was found to have rg(l) (= ra) distances (see Chapter 4) P-F =... 

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