Phosphinidene complexes

Phosphinidenes, are however, most conunonly of the m or 1X4 type (8.134). The highly symmetrical complex Nig(CO)8(p.-PPh)6 is based on a central confignration of 8 Ni atoms situated at the corners of a cube (8.134a) with a carbonyl group attached to each comer. The PPh groups are centred over each of the six faces of the cube with each P atom equidistant from four 

Ni atoms. The whole arrangement exclnding the phenyl groups, has cubic O, symmetry. Established in 1976, this was the first example of a transition metal analogue of cubane [55]. 

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Mathey et al. have described an unusual [5C+1S] process involving the reaction of a transient terminal phosphinidene complex [PhP=W(CO)5] with a bu-tadienyl carbene complex yielding a 1-phenyl-1,2-dihydrophosphine P-W(CO)5 complex [112]. 

Phosphinidenes differ from carbenes because of the additional lone pair. This lone pair enables interactions with, e.g., a transition metal group for increased stability, while maintaining carbene-hke behavior. These terminal /] -complexed phosphinidenes differ from the p2-> fi3-> and p4-complexes, which are not part of this survey. Phosphinidenes that are stabilized by a transition metal group also relate to carbene complexes. A distinction in Fischer and Schrock-type complexes has been advanced to distinguish phosphinidene complexes with nucleophilic properties from those that are electrophiHc [ 13 ]. In this survey we address this topic in more detail. 

Electrophilic and nucleophilic phosphinidene complexes have been related to the corresponding carbene complexes of which the Fischer-type is usually considered as a singlet-singlet combination and the Schrock-type as a triplet-triplet combination. However, both the strongly preferred triplet state of R-P and the M=P bond analysis suggest this schematic interpretation to be less appropriate for transition metal complexed phosphinidenes. 

The most widely employed electrophilic phosphinidene complexes are generated from M(C0)5 (M=W, Mo, Cr) complexed 7-phosphanorbornadienes 7, which were reported by Marinetti et al. in 1982 [46]. They trapped the transient phosphinidene complex 8 at 110 °C (or at 55 °C if CuCl was used as catalyst) with olefins and alkynes to obtain three-membered phosphiranes and phosphirenes, respectively. 

Cheletropic elimination of the in situ generated phosphinidene complex 8 from the 7-phosphanorbornadiene precursor is believed to be the rate-determining step before 1,2-cycloaddition occurs to the unsaturated hydrocarbon. Without catalysts these are first-order processes that depend only on the concentration of the precursor and not on that of any substrate [47]. The configu-... 

Amino-substituted phosphirane complexes undergo retro-cycloaddition at 70-90 °C to give the trappable (OC)5WPNEt2 phosphinidene complex [55]. Other substituted phosphiranes are not suited to generate phosphinidenes because they require elevated temperatures, which typically induce thermal degradation instead. 

Complexed azaphosphirenes have been used by the Streubel group to generate at 45-75 C in situ phosphinidene complex (OC)5W=PCH(SiMe3)2 (10) together with nitrilium ylid 11 [56]. 

Table 4. M=P bond distances, M=P-R bond angles, and P NMR chemical shifts of stable cationic terminal phosphinidene complexes ...

The first report on an iron complexed phosphinidene appeared in 1984 by Bertrand and coworkers [63]. They reported that abstraction of Cl" from 22 with PhjC+PFg at -90 °C presumably results in cationic phosphinidene complex 23, which was based on its downfield P NMR resonance at 954 ppm. Above -68 °C the phosphorus center inserts into a isopropyl C-H bond to give 24 as only isolable product. Dehalogenation with AICI3 resulted directly in the AICI4 salt of 24. 

The only other reaction with an aromatic substance is the C-H insertion into ferrocene [85], giving 41,which illustrates the highly electrophilic character of the phosphinidene complex. Other aromatic C-H insertions have been observed, but these likely occur by means of intermediate P,0- and P,N-ylids,such as the reaction of (0C)5W=PR withbenzophenone and azobenzene that give 42 and 43,respectively [56a, 86]. 

Using a dehydrohalogenation-ligation sequence, the group of Lammertsma generated phosphinidene complexes with group 8 (Ru, Os) and group 9 (Co, Rh,... 

The reactivity of the Cp (L)Ir=PMes (L=PPh3, CO) phosphinidene complexes is much less diverse than those with Zr. Only the formation of phosphaalkenes has been observed in the reaction with CH2I2 and CHI3 [102]. This reduced reactivity of the Ir complexes as compared to Zr complex 53 has been... 

As an illustration of the phosphorus-carbon analogy, consider the result of replacing a carbon fragment (CR2) by a phosphorus moiety (PR) in each of the common low-coordinate organic compounds (1C-4C) shown in Fig. 1. For example, the replacement of a single carbon in an alkene (1C) results in a phos-phaalkene (IP). Similarly, the phosphaalkynes (2P) are isolobal relatives of the alkynes (2C), and terminal phosphinidene complexes (3P) can be related to the... 

The formation of metal-element double bonds other than M=C should be possible by the same approach. Currently, the only example is the preparation of pt-phosphinidene complexes [MeCp(CO)2Mn]2PMes (Mes = mesityl) from [MeCp(CO)2Mn-SiMePh2] and MesPCl2 [15],... 

More than 35 years ago, it was reported that the reaction of tri-methylphosphine with the cyclotetraphosphine (CF3P)4 affords the phosphine-phosphinidene complex, Me3P —> PCF3 (43). More recently, it has been demonstrated that stable carbenes are sufficiently nucleophilic to effect the depolymerization of cyclopolyphosphines and cyclopolyarsines to produce carbene-pnictinidene complexes 66-70, the first examples of p-block pnictinidenes (44, 45). Two extreme canonical forms, 71 and 72, can be written for such species. Structure 71... 

The cyclopolyphosphines that were used to prepare the carbene-phosphinidene complexes described earlier were formed by the reduction of higher oxidation state phosphorus compounds, typically di-chlorophosphines, RPC12 (47-49). However, in some cases a separate reduction step is not necessary and it is possible to prepare the car-bene-phosphinidene complex (74) directly by reaction of a stable nucleophilic carbene with RPC12 (44). 

Note also that a related complex 98 has been synthesized by reacting a carbyne complex with a transient terminal phosphinidene complex.90... 

By writing about complexes containing triple bonds between phosphorus and transition metals, one has to take into account the triple-bond character of phosphinidene complexes which are in a nearly linear coordination mode (type C) in contrast to the usual bent coordination mode D possessing typical double-bond features. Due to the additional r-donation bonding ability of the PR moiety to the metal atom in type C and the observed bond lengths, this type of complexes has to be included into the classes of metal-phosphorus triple bond compounds. Thus, at the end of this review will appear a chapter highlighting the appropriate compounds of type C. 

Table 3 P-NMR data, M-P bond lengths, and M-P-Q bond angles of representative phosphinidene complexes ...

Figure 6.6 Illustration of the phosphinidene complex ll(MciSiN)jN Ta — PCy]. Tantalum and nitrogen atoms are shown as black spheres, silicon and phosphorus atoms are grey and carbon atoms are indicated in white shadow. The Ta=P bond length is 2. ISA...

Treatment of these phosphinidene complexes with R CHO afforded RP = C(H)R this is an alternative route to phosphaalkenes.The reaction of (Me3SiN)3N Li3 with M0CI4 or WCI4 gave [ (Me3SiN)3N MCl] (M = Mo or The molybdenum(IV) halide... 

Free 1,2,5-triphospha Dewar benzene derivative 95 is formed in two steps from 3 mol of 44a by oxidative decomplexation of the triphosphacyclohexa-l,4-diyl-2,5-diene ligand of the Hf complex 96 <1997CB1491>. An organometallic [2+1+1] cycloaddition reaction between the phosphinidene complex 97, 44a, and a coordinated CO gives access to the 1,2-diphosphacyclobutenone complex 98 (Scheme 31) <1998CEJ1917>. 

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