The substitution behavior of the organometallic complex cation [Pt{C 6 H 3 (CH 2 NMe 2 ) 2 -2,6}-(OH 2 )] + (1) has been studied with a series of nucleophiles (Cl -, Br -, I -, N 3 -, NCS -, thiourea, N, N′-dimethylthiourea, N,N,N′N′-tetramethylthiourea) as a function of nucleophile concentration ([Nu]), pH, temperature, and pressure. Complex 1 affords pseudo-first-order rate constants, k obs , for the complex-formation reaction given by k obs ) k 1 [Nu] + k -1 ; the k -1 term is due to the reverse aquation reaction and is insignificant for the stronger, S-donor nucleophiles. The mechanism for the substitution of the coordinated water molecule is associative, as demonstrated by the large negative values of ∆S q and ∆V q . The results are compared to those obtained earlier for the geometrically related neutral zwitterionic complex [Pt{C 6 H 4 (CH 2 NMe 2 )-2}(NC 5 H 4 SO 3 -3)(OH 2 )] (2). Both cyclometalated complexes 1 and 2 have very high substitution reactivities that can be ascribed to labilization which arises from the σ-bonded carbon atom of the ortho-metalated aryl ligand in combination with back-bonding to the in-plane aryl ligand that increases the electrophilic character of the Pt(II) center. In complex 1 the σ-donicity of the second chelating dimethylamine group and/or the increased chelation effect of the tridentate ligand lead to a significantly increased lability of the coordinated water molecule, compared to complex 2. The X-ray structure of the ionic complex [Pt{C 6 H 3 (CH 2 NMe 2 ) 2 -2,6}(OH 2 )] + [OSO 2 CF 3 ] -(1a) is described. The crystal structure comprises an aggregate of two complex cations and two triflate anions. The structure contains two fully equivalent square-planar coordinated Pt(II) centers with an O-ligated neutral H 2 O molecule (Pt-O ) 2.186 (2) Å) trans to the Pt-C aryl bond (Pt-C ) 1.903(2) Å); the aggregate results from the involvement of both H atoms of the two H 2 O molecules in asymmetric hydrogen bonding to O atoms of the two triflate anions.

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Electronic tuning of Ptll complexes via cyclometallation, i.e. the introduction of a Pt-C bond transto the leaving ligand, can increase their lability to that of related Pd" complexes.A. D. R, which enabled him to work at the University of WittedHerdecke.

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