The Trigonal Twist Mechanism

The trigonal twist mechanism mechanism has rarely been demonstrated in octahedral complexes with monodentate ligands. The mechanism has been demonstrated for Ru(CO)H2(PPh3)3. There is exchange between the two hydrides.

[Hydride exchange in Ru(CO)H2(PPh3)3]

The detailed pathway has been determined using EXSY. The EXSY spectrum shows cross peaks between the exchanging signals. Each hydride signal consists of a doublet of triplets of doublets. There is coupling to the unique PPh3 to produce a doublet, the two mutually trans-PPh3s and to the other hydride. During the exchange the spin state of each 31P nucleus remains the same. As a consequence the cross peak in the EXSY spectrum shows a limited number of correlations which is consistent with the trigonal twist mechanism.

The trigonal twist mechanism involves the rotation of the two hydrides and one PPh3 group around the pseudo C3 axis of the octagonal complex. Note that the mechanism involves one of PPh3 groups trans to PPh3 exchanging with the unique PPh3 group. The cross peaks of the EXSY spectrum clearly demonstrates this.

[Trigonal twist rotation applied to Ru(CO)H2(PPh3)3]

Further reading:-

G.E. Ball and B.E. Mann, "The Demonstration of the Trigonal Twist Mechanism in [RuH2(CO)(PPh3)3]", J. Chem. Soc., Chem. Commun.., 1992, 561-563.

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[Brian Mann's home page] [Department of Chemistry] [University of Sheffield]
Date:- 6th January, 1997