Phosphinidene bonding

Phosphinidenes (R-P) differ from other low-coordinate organophosphorus compounds, such as phosphaalkynes (R-C=P), phosphaalkenes (R2C=PR), and phosphaaromatics, in that the phosphorus atom carries only a single a-bonded substituent [7-9]. They relate to carbenes, nitrenes, and silylenes and likewise can exist as singlet and triplet species. The advances that led to stable carbenes [10, 11] and silylenes [12] stimulated an exploration of the chemistry of phosphinidenes. 

Table 1. Bond lengths (A) and singlet-triplet energy differences (in kcal/mol) for triplet phosphinidenes ...

Table 2. Singlet-triplet energy differences (A sx) and singlet (BDEj), triplet (BDEx), and composite (BDEsx/xs) bond dissociation energies for phosphinidenes and their W(CO)5 complexes, calculated with BP/TZP...

These composite BDE values increase strongly on substituting the phosphorus atom with n-electron donating groups. However, this substitution effect does not relate to strengthening of the M=P bond,but rather reflects a reduction in the AEji of tho froo phosphinidene or conversely an increase in the AEji for the complexed phosphinidene. Still, as a point of reference, it is noted that similar CASPT2 and BP values of 41.1 and 40.5 kcal/mol, respectively, have been deduced for the dissociation of the parent Cr complex (OC)5Cr=PH into CrfCO) and PH. 

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. 

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 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],... 

Borst et al. <2005CEJ3631> conducted a study on the synthesis of strained bicyclic phosphirane and phosphirene iron-tetracarbonyl complexes (Scheme 11). It was shown that, depending on the ring size of the resulting heterocycle, electrophilic phosphinidene [Ri-PrNP=Fe(CO)4] could be trapped intramolecularly with both double and triple bonds (compounds 146-150). The phosphinidene addition was found to be reversible at room temperature and when using phenylacetylene as solvent, exchange between phenylacetylene and the phosphinidene group took place. Compound 151 was isolated as the dimer, compound 152. 

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...

In 164, no Mn—Mn bond is present and formal double bonding exists between the Mn and Te atoms. The propensity of the 5d transition metals to form M—M bonds may be cited as a reason for these differences, and the situation is similar to the valence isomerism observed in phosphinidene (RP) and sulfenium (SR ) complexes (and their respective heavier congeners) (158-160), in which two types of structures termed open (C) and closed (D) are observed (ML = 16-electron transition metal fragment). 

Treatment of the corresponding iV-methyl(benzylidene)amine with excess of 7-phosphanorbornadiene complex 137 afforded 1,2,3-azadiphosphetidine 35 as the major product (Scheme 51) <1998HAC597>. The proposed reaction mechanism included the generation of a transient phosphinidene complex and formation of the unstable intermediate azaphosphiridine 138. Insertion of the second molecule of phosphinidene into the weak P-N bond of the three-membered heterocycle 138 afforded the 1,2,3-azadiphosphetidine 35. 

Nb and Ta derivatives are hard acids and then-complexes with P- or As-donors are limited. Tertiary phosphines, especially PMes, have been widely used to stabilize low-valent derivatives. C-H activation reactions, promoted by the formation of thermodynamically stable Ta-H, Ta-C, and Ta=C bonds have resulted in metallacycles based on unusual anionic phosphorus donors. Nucleophilic Ta phosphinidene complexes could be stabilized by a tripodal tetradentate [NN3] amido ligand. The terminal PR ligand reacts smoothly with aldehydes, providing a general synthesis of phosphaalkenes RP=C(H)R and act thus as a phospha-Wittig reactant see Phosphorus Organophosphorus Chemistry). 

Phosphinidenes RP (with a sextet of electrons at the P-atom) are isolobal with carbenes R2C, and their dimers (RP)2, with a P=P double bond, are the counterparts of the carbene dimers, the olefins R2C=CR2. The coordination chemistry of the two phosphorus species is far less developed, but the number of stable complexes is growing rapidly, as summarized in review articles. ... 

Early transition metal complexes that contain doubly bonded ligands have been developed in view to synthetical applications. Imido, phosphinidene, and chalcogenido... 

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