# _inorganics_
## 2 Results and Discussion
### Synthesis and Characterization of \(\eta^{2}\)-Pyrazolylphosphaalkene Complexes
The C\(\equiv\)O linkage is marginally elongated relative to the structurally characterized ruthenium(II) phosphaalkenyls [30], consistent with a more electron rich metal center, and is comparable to those of other established Ru(0) carbonyls, including that previously reported for **2a**. Indeed, within the bounds of uncertainty, the molecular geometry of **3d** is fully comparable to that of **2a**, with the exception of a marginally contracted P-N linkage (1.788(4) A, _cf._ 1.809(5) A **2a**), which might reasonably be attributed to the increased donor strength of pz\({}^{\text{Me2}}\) relative to pz, as is reflected in the spectroscopic features (_vide infra_). Moreover, marginally greater pyramidalization about carbon is apparent in **3d** (\(\angle\)Si-C-P 113.4(2)\({}^{\circ}\)_cf._ 116.7(3)\({}^{\circ}\) **2a**), which might similarly reflect the enhanced donor strength of pz\({}^{\text{Me2}}\) leading to an increase in retro-donation from the metal. In analogous fashion to the synthesis of **3**, treatment of phosphaalkenyls with Li[pz\({}^{\prime}\)] (pz\({}^{\prime}\) = pz\({}^{\text{CF3}}\), pz\({}^{\text{Me,CF3}}\), pz\({}^{\text{Ph}}\)) led to formation of the respective \(\eta^{2}\)-phosphaalkene complexes **4c-d**, **5c-d** (Scheme 2),  
Figure 2: Molecular structure of [Ru[\(\eta^{1}\)-\(N\)-\(\eta^{2}\)-_P_C_-P(pz\({}^{\text{Me2}}\))=CH(SiMe\({}_{2}\)C\({}_{6}\)H\({}_{4}\)CF\({}_{3}\)-_p_)](CO)(PPh\({}_{3}\))2] **(3d)** in crystals of the chloroform solvate, with thermal ellipsoids at the 50% probability level. Ancillary phenyl rings are reduced, and hydrogen atoms omitted for clarity. Selected bond distance (Å) and angles (\({}^{\circ}\)): Ru–P1 2.381(1), Ru–P2 2.359(1), Ru–P3 2.369(1), Ru–C1 2.211(4), Ru–C2 1.831(5), Ru–N1 2.229(3), P1–C1 1.782(5), P1–N2 1.778(4), C1–Si 1.860(4), C2–O 1.155(6); Ru–C1–Si 128.2(2), Ru–C1–P1 27.3(2), Ru–P1–C1 62.2(1), Ru–P1–N2 82.5(1), Ru–C1–H1 112.1(4), P1–C1–Si 113.4(2), P1–C1–H11 112.1(4), C1–P1–N2 96.1(2), P1–Ru–P2 93.6(1), P2–Ru–P3 114.0(1), P3–Ru–C1 107.1(1), P1–Ru–C1 43.5(1), P2–Ru–C2 91.1(1), P3–Ru–C2 89.1(2), C2–Ru–N1 170.2(2). 6a and 6d (Scheme 3); the SiMe\({}_{3}\) (**a**) and SiMe\({}_{2}\)Ph (**b**) derivatives of 4 and 5 have been previously described [29]. In contrast to 2 and 3, the asymmetrically substituted pyrazolates offer potential for positional isomerism; however, only a single isomer is observed in each case. Though non-trivial in lieu of crystallographic data, assignment of the specific isomer obtained can be achieved with recourse to spectroscopic data. Thus, for each of 4 and 5 a notable coupling (\(\mathrm{\text{$I$}_{FP}}\)\(\sim\)20 Hz) is apparent between the CF\({}_{3}\) substituent and the phosphacarbon only; this would imply proximity of the two moieties, strongly suggesting that the bulkier CF\({}_{3}\) substituent (_cf._ H or Me) is more favorably positioned away from the sterically encumbered metal center. This is supported by significant deshielding of the phosphacarbon center, consistent with proximity to an electron withdrawing substituent. Indeed, we find the resonant frequency of the phosphacarbon center to be very sensitive to the pyrazolyl substituents (_vide infra_), most significantly so for that at the proximal site. Assignment of the isomeric preference in 6a and 6d is a more complex undertaking, given the lack of any direct spectroscopic handle, though the same steric arguments can reasonably be applied. A more electron-withdrawing character is apparent, relative to pyrazole itself (\(\mathrm{\text{$\delta$}_{P}}\) 64.4 6a, _cf._ 58.7 2a), which conforms to expectation and is again consistent with the precedent systems, though the modest level of deshielding is inadequate to unequivocally confirm substituent proximity to the phosphacarbon. Nonetheless, we believe this to be the most likely scenario.