The preparations of two [Cr(tmpa)L]202+ dimers in which the pseudohalide ligand is a weaker -donor than NCS* are reported (tmpa = tris(2-pyridylmethyl)amine): [Cr(tmpa)(NCSe)]20(C104)2 and [{Cr(tmpa)(NCS)}2(0)-Pd(CA)](C104)2-2CH3CN; CA* 12" = 2,5-dioxo-3,6-dichloro-l,4-benzoquinone. The NCS" ligands are constrained to lie on the same side of the CrOCr bond axis by chelation of the Pd(II) center in the latter complex, contrary to the anti orientation of the parent dimer. [Cr(tmpa)(NCSe)]2OH3+ (pAfa = 1.48) and [{Cr(tmpa)(NCS)}2-(OH)Pd(CA)]3"1" (pXa = 1.15) are more acidic than [Cr(tmpa)(NCS)]2OH3+ by factors of 3.7 and 7.9, respectively.NMR spectra demonstrate that all [Cr(tmpa)L]202+ complexes reported to date have symmetric chelation by the tmpa ligands in which both apical N atoms are cis to the oxo bridge. A linear correlation of Cr(III,IV/III,III)half-wave reduction potential with p£a(Cr^-OH)Cr) pertains for both singly-and doubly-bridged (tmpa)CrOCr-(tmpa) dimers, with the exception of [Cr(tmpa)(CN)]202+. On this basis, it is concluded that the b2g HOMO energy increases with -donor effectiveness of both bridging and nonbridging ligands, in the following order: m-C6HsC02= µ-HCO2" < m-CH3C02" = NCS" < NCO" = Cl" < N3" « µ-F" < µ-". The trend toward larger pKa of Cr^-OH)Cr conjugate acids with increasing substituent Lewis basicity shows that CrOCr -bonding strength is highly sensitive to the competition between µ-2" and µ-' or L" groups for donation to the metal. In this way, both the reduced oxo group basicities and the diamagnetism of the NCSe* and NCS{Pd(CA)/2}" derivatives may be understood.

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Kinetic studies of [(tmpa)Cr(M-0)(ti-RC02)Cr(tmpa)]3+ aquation and hydroxide-assisted hydrolysis reactions 1 and 2 are reported (tmpa = tris(2-pyridylmethyl)amine; R = H, CH3, CH2CI, CHCI2, C(CH3)3, CPÍ13,1-adamantyl, Ph-4-X (X = H, OH, NMe2, OCH3, CH3, CF3, N02, F, Cl)): [(tmpa)Cr(M-0)(w-RC02)Cr(tmpa)]3+ + H20 -[(tmpa)Cr(w-OH)2Cr(tmpa)]4+ + RCOf (1); [(tmpa)Cr(a-0)(M-RC02)Cr(tmpa)]3+ + 20H" + H20 -2[Cr-(tmpa)(OH)2]+ + RCO2" (2). A linear Hammett plot of acid-independent aquation rate constants (k^) gives = +0.30, while the corresponding plot of log(fcaq) vs -pXa(RCOOH) for R = H, CH3, CH2CI and CHC12 complexes exhibits a slope of +0.10; reactivities of carboxylate-bridged complexes with sterically-demanding R groups (CPI13, C(CH3)3, 1-adamantyl) are smaller than anticipated on the basis of RCO2" basicity alone.

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