markable that such a strongly bound ligand would be so reactive, it is possible to reconcile these seemingly contradictory observations. The key to this is the extra resonance stabilization experienced by the aromatic cyclopentadienyl unit, which may be regarded as a factor imparting a greater relative tendency to resist change. This readily translates into both a greater reluctance of this ligand to undergo disruption of its x system through coupling reactions (which are, however, known in some cases1"]) and a weaker metal-ligand bonding interaction. Thus, the lower reactivity of the cyclopentadienyl ligand in this case is a reflection of its inherent stability, rather than the strength of its binding to the metal center. However, if one were to deal with a process involving the complete removal of the 71 ligands, it would be expected that the cyclopentadienyl unit should be lost more readily.1121 Additional experimental and theoretical studies are underway to gain further insight into these observations.
Experimental Procedure
Zb, L = PEtI: A blue-green solution of 1.0 g (3.1 mmol) 1 in 50 mL of T H F was cooled under nitrogen to -78°C and 0.90 mL (6.2 mmol) of PEt3 was added, yielding a dark-blue solution. To this solution was slowly added 0.9 g (6.6 mmol) of the potassium salt of the 2,4-dimethylpentadienyl anion dissolved in 50 mL of THF, after which the mixture was stirred for 2 h at -78"C, resulting in a brown solution. The reaction mixture was allowed to warm to room temperature and stirred for about 6 h. The solvent was removed in vacuo and the crude product extracted with several portions of hexane and filtered on a coarse frit. The greenish-brown hexane solution was concentrated under reduced pressure and cooled to -78"C, yielding a red, air-sensitive crystalline product. Complete IR data (Nujol): 5 [cm-']=3170 (m), 1563 (m), 1304 (mw), 1261 (s), 1170 (sh), 1155 (mw), 975 (w). 889 (w), 797 ( s ) . -' H -N M R : ~= ~. O ~( ~, ~H ) , ~. O ~( ~, ~H , J = I Hz), 1.83(s,8H),1.26(m, 6H), 0.71 (m, 9H), -0.85 (t, 2H, J = 6 Hz).-'"C-NMR: 6=112.46 (d, I C , J = 156 Hz), 101.37 (s, 2C), 100.60 (d, 5'2, J = 170 Hz), 53.89 (t, 2C, J= 150 Hz),30.91 (4, 3C, J = 1 3 4 Hz), 18.69(t, 2'2, J=126 Hz), 7.94(q, 2C, J=128 Hz).--"P-NMR: 6=41.28.-Correct C,H analysis.
3 :
To a solution of 0.40 g (1.22 mmol) Zb, L = PEt3, in 30 mL of hexane at room temperature was added 0.20 mL (3.6 mmol) of CH,CN dropwise. Reaction began immediately with the formation of a dard-red solution, followed by the precipitation of an oily dark-brown solid. Upon completion of the addition, the dark-red reaction mixture was stirred for an additional 30 min. Cooling the solution to -78°C for ca. 12 h yielded a dark-brown airsensitive product. After removing the supernatant solution, the crystalline product was dried in vacuo. Complete IR (Nujol): i.[cm-']=3170 (m). 1629 (s), 1563 (m). 1311 (mw), 1261 (ms), 1172 (mw), 1157 (m). 977 (w), 890 (m), 800 (5) cm-'.-'H-NMR: 6=0.89 (d, I H , J = 5 Hz), 1.16 (s, 3H), 1.31 (s, 3H), 1.52 (d, I H , J = 5 Hz), 2.04 (s, 3H), 2.17 (d, J = 2 0 Hz), 2.76 (d, l H , J = 2 0 Hz), 5.23 (s, 1 H), 5.89 (s, 5 H ) . -' C N M R : d=21.93 (9, I C , J = 1 2 2 Hz), 26.37 (q, I C, J = 120 Hz), 26.76 (4, 1 C, J= 125 Hz), 60.58 (t, 1 C, J= 124 H ~) , 6 9 . 9 7 ( t , l C , J = 1 3 6 H ~) , 7 6 . 5 3 ( ~, l C ) , 111.74(d,SC,J=l70Hz), 122.33 (d, 1 C, J = 158 Hz), 133.98 (s, 1 C), 174.74 (s, 1 C).-Correct elemental analysis (C, H, N, Ti).