Cyanide exchange protonated complexes

The cyanide exchange on [M(CN)4]2 with M = Pt, Pd, and Ni is a rare case in which mechanistic comparisons between 3d, 4d, and 5d transition-metal complexes. Surprisingly, the behavior of these metal square-planar centers leads to mechanistic diversity involving pentacoordinated species or transition states as well as protonated complexes. The reactivities of these species are strongly pH-dependent, covering 15 orders of magnitude in reaction rates.85... 

A further interesting observation from the cyanide exchange study relates to the fact that the exchange rate constants for the equatorial cyanide in the pentacyano complexes compared to the protonated spe-... 

The different exchange processes described in Sections III-V can be combined to illustrate the reactivity of the different sites in these oxo cyano complexes as a function of pH and the possible interdependence thereof. The three processes that are compared are the inversion along the O-M-O axis (illustrated in Fig. 15 related to proton exchange), the oxygen exchange, and the cyanide exchange. 

In summary, it is clear from the above-discussed aspects that it was possible by multinuclear NMR (oxygen-17, nitrogen-15, carbon-13, and technetium-99) to successfully study the very slow cyanide exchange and the slow intermolecular oxygen exchange in these oxocy-ano complexes and correlate them both with the proton-transfer kinetics. Furthermore, the interdependence between the proton transfer and the actual dynamic inversion of the metal center was clearly demonstrated. 

Oxo cyano—MoIV complexes are well known and their chemistry has been extensively reviewed.652,724,725 Recent work has included the measurement of the proton, oxygen, and cyanide exchange rates for archetypal (all trans) [Mo02(CN)4]4, [MoO(OH)(CN)4]3, and... 

The equilibria governing the complex formation and oxygen and proton exchange in these systems are given by Scheme 1, where CN denotes the total free cyanide, i.e., HCN/CN-, and is used as such throughout this chapter. In the complex formation in Eq. (2) X represents different entering nucleophiles such as NCS , F , CN-, and pyridine (py). 

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