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.
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.
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, pp. 561-563.