DICHROMASTIBACUMULENIUM IONS [ETA-6-AREN(CO)2CR=SB=CR(CO)2AREN-ETA-6]- STABILITY AS FUNCTION OF ETA-6-BOUND AREN LIGANDS ON THE CHROMIUM ATOM( )

Abstraction of chloride by Lewis-acid reagents from the stibinidene co mplexes [(eta6-arene(CO)2Cr}2SbCl] (arene = Me6C6: 1a; 1,3,5-Me3C6H3: 2a; C6H6: 3a) produced 4-6, the allene type dichromastibacumulenium io ns [arene(CO)2Cr=Sb=Cr(CO)2arene]+, which were detected spectroscopica lly in solution. Their stability increases with an increasing number o f methyl substituents at the arene ligand: salts with cation 6 (arene = C6H6) Could not be isolated as such, while salts with 5 (arene = mes ) or 4 (arene = hmb) could. However, 6 can be trapped with chelate bas es L2 (L2 = 2,2'-bipyridine = bpy, 1,10-phenanthroline) yielding [{C6H 6(CO)2Cr}2CrSb(L2)]Y (7-9), while 5 and bpy gave unclear results and 4 is effectively inert towards bpy (IR, variable temperature UV/vis spe ctroscopy). 4-6, in reverse of their formation, react instantaneously with anionic nucleophiles X- to give stibinidene complexes [{arene(CO) 2Cr}2SbX] identified spectroscopically in solution. Investigation of t he crystal structure of [(C6H6(CO)2Cr}2Sb(bpy)]PF6 . 1/2bpy, 7b . 1/2b py, indicated relatively long Sb-N bonds suggesting that the base is o nly weakly bound to the Sb+ centre. These findings are in line with fi rstly the ease of decomposition of 7b in solution, spectroscopy sugges ting that [{C6H6(CO)2Cr}2SbF] is formed, and secondly the ready displa cement by anionic nucleophiles X- of the chelate base, where stibinide ne complexes, 3, are formed. 7, 8 are easily accessible by means of a salt metathesis from [{C6H6(CO)2Cr}2SbX'] (X'= Cl, Br) and NaBPh4 or K PF6 in the presence of L2. In this manner 4, 5 - BPh4 were synthesized as well and isolated as analytically pure microcrystalline salts. The enhanced stability of 4 and 5 compared with 6 becomes apparent in the ir formation from la and 2a, respectively, and SbCl3-1a yields 4. ''[S b2Cl7]'' in an equilibrium reaction with two equivalents of SbCl3 (upo n cooling the equilibrium is slightly shifted back towards the educts) ; generating 5 from 2a requires 100-150 equivalents of SbCl3, and 3a d oes not react with SbCl3 under formation of 6. Thus, for cations [L(n) M=Sb=ML(n)]+ the following order of stability can be set up: L(n)M=Me6 C6(CO)2Cr > 1,3,5-Me3C6H3(CO)2Cr much greater than C6H6(CO)2Cr almost- equal-to Cp(CO)2Mn>CP'(CO)2Mn, while the stability of the stibinidene complexes [{L(n)M}2SbX] increases from left to right. Steric factors are at least contributory to the low reactivity of [Me6C6(CO)2Cr=Sb=Cr (CO)2C6Me6]+. Complex fragments (eta6-arene(CO)2Cr} proved to the suit able for the stabilization of strongly pi-acidic, low coordinated liga nds and may in special cases be markedly superior even to (CP(CO)2Mn} .