Allyl complexes electronic spectra

The transition-metal allyl complexes are air- and temperature-sensitive solids Cr(allyl)3, m.p. ca. 70° Ni(allyl)2, m.p. ca. 0°. The infrared spectrum of both compounds indicates that the bonding of the allyl group to the metal involves r electrons (the olefinic bond appearing at 1520 and 1493 cm.-1, respectively) they can be identified by their mass spectra. 

Formula II shows one trans-double bond which is shared with the nickel atom. Furthermore, there are six carbon atoms which are in the state of an -hybridization. Each C atom shares one r-electron with the nickel. (The complex shows the correct molecular weight for NiCi2Hig, and there is no absorption in the infrared spectrum characteristic of double bonds.) This formulation has some relationship to structures which have been recently proposed by different authors (2, 4) for various allylic groups bonded to transition metal carbonyls. 

Titanium group allyl compounds are sensitive to air, water, and alcohols. Tetrallyl derivatives of zirconium and hafnium are dynamic compounds. The complex Hf(allyl)4 gives the AX4 type NMR spectrum even at very low temperatures (199 K). The Ti(III) complexes are paramagnetic magnetic moments correspond to one unpaired electron and, for various allyl groups, vary in the 1.47-1.75 BM range. The properties of allyl... 

Wilke and Bogdanovid (1961) first prepared bis(n-allyl)nickel(0) (X) from allylmagnesium bromide and nickel(II) bromide. The 16-electron complex is yellow and quite volatile. Its nmr spectrum suggests the planar delocalized structure for the allylic ligand (Wilke et al., 1966 Bonnemann et al., 1967). X-Ray analysis of the complexes established the sandwich arrangement (Wilke and Bogdanovic, 1961 Dietrich and Uttech, 1963,1965). A monograph written by Jolly and Wilke (1974, 1975), as well as a review by Fischer et al. (1973), describe characteristic features of the nickel complexes. The synthetic utility of these complexes has been reviewed by Heimbach et al. (1970). 

Nitric oxide, like the 7r-allyl group, can act as a 3-electron ligand a number of stable a -cyclopentadienyl nitrosyl complexes are known. The preparations of some of these complexes are shown in Figures 30 and 32. The series tt-CsHsMNO, where M = Ni [28], Pd [194], and Pt [195], is well characterized and the microwave spectrum of the nickel complex shows the synametric structure given in Figure 28, p. 112 [196]. In this molecule, the short Ni-N distance (1-676 0-02 A) indicates considerable multiple bonding. The dinitrosyl n -C5H r(NO)2CI similarly has a short Cr-N bond (1-72 A) [196a]. 

---