Associative substitution occurs by the addition of a ligand to a metal complex followed by dissociation of an original ligand. Associative pathways are not typically seen in 18-electron complexes due to the requisite intermediates having more than 18 electrons associated with the metal atom. 18 electron indenyl complexes; however, have been shown to undergo substitution via associative pathways quite readily. This is attributed to the relative ease of η5 to η3 rearrangement due to stabilization by the arene. This stabilization is responsible for substitution rate enhancements of about 108 for the substitution of indenyl complexes compared to the corresponding cyclopentadienyl complex.
Kinetic data support two proposed mechanisms for associative ligand substitution. The first mechanism, proposed by Hart-Davis and Mawby, is a concerted attack by the nucleophile and η5 to η3 transition followed by loss of a ligand and a η3 to η5 transition.
η5 to η3 rearrangement in other ligands
Indenyl like effects are also observed in a number of non indenyl substituted metal complexes. In fluorenyl complexes, associative substitution is enhanced even further than indenyl compounds. The substitution rate of Mn(η5-C13H9)(CO)3 is about 60 times faster than that of Mn(η5-C9H7)(CO)3
Veiros conducted a study comparing the rate of substitution on [(η5-X)Mn(CO)3] where X is cyclopentadienyl, indenyl, fluorenyl, cyclohexadienyl, and 1-hydronaphthalene. Unsurprisingly, it was found that the ease of η5 to η3 haptotropic shift correlated to the strength of the Mn-X bond.
The indenyl effect is a term given by Fred Basolo to a phenomenon first reported by Adam J. Hart-Davis and Roger J. Mawby in 1969. Hart-Davis and Mawby found that the rate of conversion of (η5-C9H7)Mo(CO)3CH3 to the phosphine-substituted acetyl complex followed bimolecular kinetics. This rate law was attributed to the ability of the indenyl ligand to undergo a η5 to η3 haptotropic rearrangement, which abets associative attack on the metal. The corresponding reaction of tributylphosphine with (η5-C5H5)Mo(CO)3CH3 was 10 times slower.
Subsequent work by Hart-Davis, Mawby, and White compared CO substitution by phosphines in Mo(η5-C9H7)(CO)3X and Mo(η5-C5H5)(CO)3X (X = Cl, Br, I) and found the cyclopentadienyl compounds to substitute by an SN1 pathway and the indenyl compounds to substitute by both SN1 and SN2 pathways. Mawby and Jones later studied the rate of CO substitution with P(OEt)3 with Fe(η5-C9H7)(CO)2I and Fe(η5-C5H5)(CO)2I and found that both occur by an SN1 pathway with the indenyl substitution occurring about 575 times faster. Hydrogenation of the arene ring in the indenyl ligand resulted in CO substitution at about half the rate of the cyclopentadienyl compound.
Work in the early 1980s by Fred Basolo found the SN2 replacement of CO in Rh(η5-C9H7)(CO)2 to be 108 times faster than in Rh(η5-C5H5)(CO)2. Shortly afterwards, Basolo tested the effect of the indene ligand on Mn(η5-C9H7)(CO)3, the cyclopentadienyl analogue of which having been shown to be inert to CO substitution. Mn(η5-C9H7)(CO)3 did undergo CO loss and was found to substitute via an SN2 mechanism.
- C. White; R.J. Mawby; A.J. Hart-Davis (1970). "Reactions of tricarbonyl-π-cyclopentadienylhalomolybdenum(II) complexes with phosphorus(III) ligands: a kinetic study". Inorg. Chim. Acta. 4: 441–6. doi:10.1016/S0020-1693(00)93323-1.
- Fred Basolo (1982). "Associative substitution reactions of 18-electron transition metal organometallic complexes". Coord. Chem. Rev. 43: 7. doi:10.1016/S0010-8545(00)82089-5.
- Mark E. Rerek; Liang-Nian Ji; Fred Basolo (1983). "The indenyl ligand effect on the rate of substitution reactions of Rh(η-C9H7)(CO)2 and Mn(η-C9H7)(CO)3". J. Chem. Soc., Chem. Commun. (21): 1208–09. doi:10.1039/C39830001208.
- Mark E. Rerek; Fred Basolo (1984). "Kinetics and mechanism of substitution reactions of η5-cyclopentadienyldicarbonylrhodium(I) derivatives. Rate enhancement of associative substitution in cyclopentadienylmetal compounds". J. Am. Chem. Soc. 106 (20): 5908. doi:10.1021/ja00332a026.
- Luis F. Veiros (2000). "The Role of Haptotropic Shifts in Phosphine Addition to Tricarbonylmanganese Organometallic Complexes: the Indenyl Effect Revisited". Organometallics. 19 (16): 3127. doi:10.1021/om000195j.
- M.J. Calhorda; Romão, Carlos C.; Veiros, Luis F. (2002). "The Nature of the Indenyl Effect". Chem. Eur. J. 8 (4): 4. doi:10.1002/1521-3765(20020215)8:4<868::AID-CHEM868>3.0.CO;2-I.
- Hart-Davis, A.J.; Mawby, R.J. "Reactions of -indenyl complexes of transition metals. Part I. Kinetics and mechanisms of reactions of tricarbonyl--indenylmethylmolybdenum with phosphorus(III) ligands". J. Chem. Society A. 1969: 2403–6. doi:10.1039/J19690002403.