# _inorganics_
## 2 Results and Discussion  
### Synthesis and Characterization of \(\eta^{2}\)-Pyrazolylphosphaalkene Complexes  
As we have previously described [29; 40], the ruthenaphosphaalkenyls **1c-d** were prepared via hydoruthenium of the respective phosphalkynes RMe\({}_{2}\)SiC\(\equiv\)P with [RuHCl(CO)(PPh\({}_{3}\))\({}_{3}\)] (Scheme 2). Each system is readily identified on the basis of characteristic spectroscopic signatures, _viz._ (i) the heavily deshielded \({}^{31}\)P[\({}^{1}\)H]-NMR resonance for the phosphaalkenyl moiety; (ii) consistently integrating resonances for two equivalent PPh\({}_{3}\) ligands; (iii) HMBC correlations (\({}^{1}\)H-\({}^{29}\)Si; \({}^{1}\)H-\({}^{31}\)P) confirming the presence of the silyl group and alkenic proton; (iv) infrared band for the retained carbonyl (\(\
u_{\rm CO}\) 1930-1950 cm\({}^{-1}\)), characteristic of a ruthenium(II) carbonyl complex. The reactions of **1c-d** with Li[pzMe2] each afford a single complex (**3c-d** respectively), which is spectroscopically comparable to those obtained similarly from **1a** and **1b**[29; 30]. Thus, the characteristic resonances for the phosphaalkenyl and PPh\({}_{3}\) ligands are lost, being replaced by three new, mutually coupling, resonances (1:1:1 ratio) in the region 50-30 ppm, which is commonly associated with saturated (\(\lambda^{3}\sigma^{3}\)) phosphorus centers. In each case, the lower frequency resonance is identified as that associated with the phosphacarbon, on the basis of a \({}^{1}\)H-\({}^{31}\)P-HMBC correlation to the, now appreciably shielded, \"CH(SiMe\({}_{2}\)R)\" moiety (identified from \({}^{1}\)H-\({}^{29}\)Si-HMBC spectra and a significant \({}^{1}\)J\({}_{\rm PC}\) coupling (-70 Hz)). The latter is observed at \(\delta_{\rm H}\) -1.7 and \(\delta_{\rm C}\) -42, which, though appreciably lower frequency than for a classical alkenyl \"CH\", remains somewhat deshielded for a fully saturated \"alkyl\" moiety. Indeed, as was previously noted for **2a-b** and **3a-b**, the magnitude of the \({}^{1}\)J\({}_{\rm CH}\) coupling (-130-140 Hz) is intermediate between those of C\({}_{2}\)H\({}_{4}\) (156 Hz) and CH\({}_{4}\) (125 Hz) [41], while \({}^{1}\)J\({}_{\rm PC}\) are essentially unperturbed from those of the parent phosphaalkenyls, consistent with the case of a \(\pi\)-bound phosphaalkene. This scenario is further supported by a significant reduction in \(\
u_{\rm CO}\) for the retained carbonyl (1900-1910 cm\({}^{-1}\)), indicative of increased density at metal and, on the basis of calculated force constants, consistent with reduction of the metal to Ru(0) [42; 43; 44; 45; 46]. Though augmented by computational data, the direct structural characterization of \(\eta^{2}\)-pyrazolylphosphaalkene complexes has been limited previously to the single crystal X-ray structure of pyrazole derivative **2a** and a low-quality structure of **3a** that served to confirm connectivity [30]. However upon prolonged standing at ambient temperature, a concentrated CDCl\({}_{3}\) solution of **3d** yielded X-ray quality single crystals of the chloroform solvate (Figure 2). In common with the precedent structure of **2a**, compound **3d** exhibits distorted trigonal-bipyramidal geometry about ruthenium, with PPh\({}_{3}\) ligands and the \(\eta^{2}\)-phosphaalkene lying in the equatorial plane. The carbonyl adopts an axial position, along with the metal-pyrazolyl bond, the latter being marginally distorted from linearity (\(\angle\)C\({}_{\text{CO}}\)-Ru-N 170.2(2)\({}^{\circ}\)) due to the strain of bridging the Ru-phosphacarbon linkage. Internal angles for the phosphacarbon moiety indicate partial pyramidalization of the carbon center, while the unusually tight angles about phosphorus can be attributed to the constraint of the bridging pyrazyl group. Taken together with the P-C linkage 1.782(5) A, which is intermediate between a single and double bond [47; 48], these data are wholly consistent with our previous conclusion of \(\eta^{2}\)-binding, rather than a discrete metallacyclic species.