Metal-metal multiply bonded complexes of technetium .5. Tris and tetrakis(formamidinato) complexes of ditechnetium

Compounds of the type Tc2Cl4(PR(3))(4) (PR(3) = PEt(3), PMe(2)Ph, PMePh(2)) react with the molten formamidines HDPhF (HDPhF = diphenylformamidine) and HDTolF (HDTolF = di-p-tolylformamidine) to produce mixtures of tris- and tetrakis-bridged formamidinate complexes of ditechnetium. The displacement of chloride and phosphine by [DPhF](-) was accompanied by the oxidation of the dimetal core to produce the mixed-valent complexes Tc-2(DPhF)3Cl(2) (1) and Tc-2(DPhF)(4)Cl (2) in modest yield. The solid-state structures of the di-p-tolyl analog of 1, Tc-2(DTolF)(3)Cl-2 (1a), and Tc-2(DPhF)(4)Cl . C7H8 (2 . C7H8) have been determined by single crystal X-ray diffraction studies and are described in detail. The structure of 1a consists of three formamidinate ligands spanning the two technetium atoms. The two chloride ligands, which complete the coordination sphere, are bound equatorially at distances of 2.357(1) and 2.346(2) 8, from the metals. Though possessing no crystallographic symmetry, 1a approximates C-2 nu symmetry. The metal-metal bond length of 2.0937(6) Angstrom ranks among the shortest reported for technetium and is indicative of a Tc-Tc multiple bond. Compound 2 crystallizes with the Tc atoms colinear with a crystallographic 4-fold axis. The four bridging formamidinate ligands are arranged in a lantern geometry about the dimetal unit. The chloride is bonded in an axial position at a distance of 2.450(4) Angstrom. The Tc-Tc bond length of 2.119(2) Angstrom is also consistent with the presence of a high order Tc-Tc bond. The electronic structures of 1 and 2 were investigated by means of SCF-X alpha-SW molecular orbital calculations using the model compounds Tc-2(HNCHNH)(3)Cl-2 and Tc-2(HNCHNH)(4)Cl. The results support the presence of a sigma(2) pi(4) delta(2) delta* ground state configuration giving rise to a formal bond order of 3.5. The LUMO in both cases is a low-lying pi* orbital. The formamidinate complexes 1 and 2 have been further characterized by IR spectroscopy and cyclic voltammetry. The crystallographic parameters for 1a and 2 . C7H8 are as follows: Tc-2(DTolF)(3)Cl-2 (1a), monoclinic space group P2(1)/n (No. 14) with a = 16.185(2) Angstrom, b = 15.637(2) Angstrom, c = 17.812(1) Angstrom, beta = 110.142(5)degrees, V = 4232.3(6) Angstrom(3) and Z = 4; Tc-2(DPhF)(4)Cl . C7H8 (2 . C7H8), tetragonal space group P4/ncc (No. 130) with a = 15.245(2) Angstrom, c = 21.832(3) Angstrom, V = 5074.1(9) Angstrom(3) and Z = 4.