INFRARED SPECTROSCOPIC STUDY OF THE PHOTOCHEMICAL SUBSTITUTION AND OXIDATIVE ADDITION-REACTIONS OF (ETA(5)-C5R5)M(CO)4 COMPOUNDS OF THE GROUP-5 METALS - CHARACTERIZATION OF THE PRODUCTS OF REACTION WITH N2, H-2, AND HSIET3-XCLX AND THE KINETIC INVESTIGATION OF (ETA(5)-C5R5)M(CO)3 INTERMEDIATES

IR spectroscopy has been used to study the photochemical reactions of H-2 and N2 with CpM(CO)4 and Cp*V(CO), (Cp = eta5-C5H5, Cp* = eta5-C5Me5; M = V, Nb, and Ta) and of HSiEt3-xClx (x = 0, 2, and 3) with CpV(CO)4. The reactions have been studied by FTIR in liquid xenon solution (lXe) at ca. -80-degrees-C and by time-resolved IR spectroscopy (TRIR) in n-heptane solution at room temperature. In nearly all cases, UV irradiation leads to substitution of only one CO group. The only exceptions were the reactions of N2 with CpNb(CO)4 in lXe and with CPV(CO)4 in solid matrices at 20 K, where CpM(CO)2(N2)2 species were generated as secondary photoproducts. Reaction with H-2 led to formation of nonclassical dihydrogen complexes CpV(CO)3(eta2-H-2), Cp*V(CO)3(eta-H-2) and CpNb(CO)3(eta2-H-2), which was in thermal equilibrium with the classical dihydride CpNb(CO)3H2. Nu(H-H) IR bands have been observed for all three dihydrogen complexes. Reaction of CpTa(CO)4 with H-2 led to oxidative addition and formation of CpTa(CO)3H2, which decayed over a period of 30 min in supercritical xenon solution at room temperature. IR data were also obtained for reaction with D2 and, in the case of V, with HD. IR spectra suggest that reaction of CpV(CO)4 with HSiEt3 results in the arrested oxidative addition to form a labile CpV-(CO)3(eta2-H-SiEt3) complex, the first example of this type of compound of a group 5 metal. By contrast, reaction with HSiCl3 and HSiEtCl2 led to full oxidative addition, CpV(CO)3(H)SiR3. TRIR measurements showed that formation of all of these CpM(CO)3L species proceeds via a dissociative mechanism with transient formation of CpM(CO)3. CpV(CO)3 is ca. 100 times more reactive than its Nb and Ta analogs and nearly 1000 times more reactive than CpMn(CO)2 under similar conditions. Cp*V(CO)3 is even more reactive. Photoacoustic calorimetry (PAC) has been combined with TRIR to estimate V-(N2) and V-(eta2-H-2) bond dissociation enthalpies. The PAC results suggest that CpV(CO)3 interacts with the n-heptane solvent and is probably more correctly formulated as CpV(CO)3...(n-heptane). Finally, the reactions of V(CO)6 with N2 and with H2 were studied to compare the behavior of d5 vanadium. IR evidence was found for the formation of V(CO)5(N2) and V(CO)5(eta2-H-2) both in lXe and in n-heptane (TRIR). These compounds were significantly shorter lived than the corresponding CPV(CO)3L species under similar conditions. The photochemical formation of V(CO)5L occurred via V(CO)5, detected by TRIR, which was significantly less reactive toward CO, N2, and H-2 than was CPV(CO)3.