In contrast to the anion radicals of 1, lO-phenanthroline (phen) or its 4,7-dimethyl derivative (dmphen) with their 2B1 ground state, the 3,4,7,8-tetramethyl derivative tmphen'-has a 2A2 ground state as evident from EPR/ENDOR measurements. The small n: spin population at the potentially metal-coordinating nitrogen centres in the 2A2 species as opposed to the 2B, state causes distinct spectroscopic effects in corresponding diorganoplatinum(I1) complexes. The neutral compound (phen)PtMes,, Mes = mesityl, has been structurally characterized as a square planar PtII complex with tightly protected axial sites: monoclinic, space group P2,/c, 2 = 4, a = 9.378(1), b = 15.791(2), c = 16.581(1) A, p = 103.18(1)". All complexes (NAN)PtMes2, N A N = bpy (2,2'-bipyridine), phen, tmphen, dppz (dipyrido[3,2-a : 2',3'-clphenazine) and pdo (1,lOphenanthroline-5,6-dione) are oxidized to EPR-silent platinum(Ir1) species and reduced to anion radical complexes. The latter clearly contain Pt" and heterocyclic anion radical ligands; however, they exhibit a trichotomous EPR behaviour. Whereas the radical complex [(phen)PtMes2]' -shows a rather low gi,,-value of 1.983 and distinct g anistropy in frozen solution like the analogous complex of bpy' -, the formally related [(trnphen)PtMes,]' -has an isotropic g-factor (2.0030) much closer to the free-electron value and a distinctly smaller g anisotropy. The phenanthroline-derived radical complexes [(pdo)PtMes,]*-and [(dppz)Pt-Mes,]'-exhibit virtually no contribution from the coordinated heavy metal to the singly occupied MO (b,) which is mainly centred at the heteroatoms outside the phenanthroline n: system. 163.

The gallium(I) compound Ga4[C(SiMe3)3]4 (1) with a tetrahedral Ga4 core in the solid state gives on evaporation the monomeric Ga(I) alkyl Ga[C(SiMe3)3] with the unique structural situation of a Ga atom solely coordinated by one singly bonded carbon ligand. Its molecular structure was determined now by gas-phase electron diffraction yielding a Ga−C bond length of 206.4(17) pm, similar to that found in the solid state for the tetramer and much longer than in compounds with gallium in the normal oxidation state of +3. As shown by cyclic voltammetry, 1 and the analogous cluster In4[C(SiMe3)3]4 (2) could both be reversibly reduced at about −2 V vs Fc+/0. Whereas the reduced organoindium tetramer did not exhibit any detectable EPR signals between 4 and 300 K, the gallium analogue showed an EPR spectrum compatible with the formulation [(RGa0.75) 4]•-; i.e. the unpaired electron was found to be delocalized over all four metal centers (a(69Ga) = 1.93 mT, a(71Ga) = 2.45 mT, 4 Ga).

The oxidation and reduction behavior of complexes [(mpz)M(CN)(5)](2)(-) between the group 8 pentacyanometalates(II) and the N-methylpyrazinium ion mpz(+) was studied using spectroelectrochemical techniques (UV/vis/NIR, IR, EPR) in nonaqueous and, in part, aqueous media. As a further source of information on the electronic structure, we studied the variable solvatochromism of MLCT absorption bands in the precursor ions [(mpz)M(CN)(5)](2)(-). The mpz-centered one-electron reduction showed a tendency for concomitant loss of cyanide (EC process) to yield ions [(mpz)M(CN)(4)](2)(-), the lability increasing in the order M = Os << Ru < Fe. In contrast to the EPR spectra obtained from reduction with S(2)O(4)(2)(-) in aqueous media, which showed no evidence for close mpz(*)/metal association, the intra muros electrolysis experiments in DMSO or acetonitrile yielded the following results: The EPR spectrum that was previously assigned to [(mpz)Fe(CN)(5)](3)(-) is now ascribed to [(mpz)Fe(CN)(4)](2)(-). Whereas the dissociatively inert osmium radical complex [(mpz)Os(CN)(5)](3)(-) is distinguished by a broad EPR signal with a rather large value g(iso) = 2.0157, the EPR spectrum of [(mpz)Ru(CN)(4)](3)(-) reveals well-resolved separate coupling with the (99)Ru and (101)Ru isotopes in natural abundance. Spectroelectrochemistry and reactivity patterns of the complex ions [(mpz)M(CN)(5)](2)(-) are reminiscent of results for corresponding nitrosyl complexes [(NO)M(CN)(5)](2)(-), which suggests that the mpz(+)/mpz(*) ligand redox pair can function as an organic analogue of the NO(+)/NO(*) redox system.

The tetranuclear complexes {(&mgr;(4)-TCNX)[Ru(NH(3))(5)](4)}(A)(8) and (&mgr;(4)-TCNX)[Mn(CO)(2)(C(5)Me(5))](4) [A = PF(6) or CF(3)SO(3); TCNX = TCNE (tetracyanoethene), TCNQ (7,7,8,8-tetracyano-p-quinodimethane), or TCNB (1,2,4,5-tetracyanobenzene)] were studied by variable-temperature (2-300 K) SQUID susceptometry. Mono- and dinuclear species [(PhCN)Ru(NH(3))(5)](PF(6))(2) (PhCN = benzonitrile) and {(&mgr;-L)[Ru(NH(3))(5)](2)}(PF(6))(4) (L = 1,4-dicyanobenzene (terephthalodinitrile) or pyrazine) were also investigated for comparison and were found to be essentially diamagnetic. Despite the even electron count, both the ruthenium and manganese tetranuclear complexes are paramagnetic, albeit with different spin-spin exchange coupling patterns. The manganese systems are characterized by exchange-coupled S = 1 states at the individual metal centers, whereas the magnetic behavior of the tetranuclear ruthenium compounds results from an exchange-coupling interaction between two S = (1)/(2) sites, identified as Ru(III)/Ru(II) mixed-valence pairs.

The room-temperature reactions of closo- exo-4,5-[(µ-H) 2 Li(TMEDA)]-1-Li(TMEDA)-2,3-(SiMe 3 ) 2 -2,3-C 2 B 4 H 4 with anhydrous LnCl 3 (Ln ) Sm, Gd, Dy, Ho, Er), in molar ratios of 2:1 in dry benzene (C 6 H 6 ), produced the sandwich paramagnetic species [Li(TMEDA 1), Gd (2), Dy (3), Ho (4), Er ( 5)) in yields of 65%, 88%, 71%, 90%, and 93%, respectively. The compounds were characterized by IR spectroscopy, magnetic susceptibility measurements, and single-crystal X-ray diffraction studies. Compounds 2 and 3 were further characterized by EPR spectroscopy, and compound 1 was also characterized by 1 H, 13 C, 11 B, and 7 Li NMR spectroscopy. All compounds are bent-sandwich complexes in which a Ln(III) atom is coordinated to two carborane ligands and two chloride ions. The room-temperature magnetic susceptibilities of 1-4 were found to be 1.1, 7.3, 9.4, and 10.1 µ B per metal atom, respectively. Experimental SectionMaterials. Benzene, N,N,N′,N′-tetramethylethylenediamine (TMEDA), and n-hexane were dried over NaH and Na metal and doubly distilled before use. All other solvents were dried over 4-8 Å molecular sieves (Aldrich) and were either saturated with dry argon or degassed before use. The syn