Reduction–substitution reactions of [TcO₄]^– with (o-aminophenyl)diphenylphosphine (HL) at a strictly controlled stoichiometric metal/ligand ratio gave neutral oxotechnetium(V) complexes of the type [TcOL₂(OR)](R = Me 1 or Et 2). Analogous [ReOL₂(OR)] species (R = H 3, Me 4, Et 5, Pr 6, Bu 7, C₂H₄OH 8, C₃H₆OH 9, Ph 10 or OCMe 11) are instead produced via ligand-exchange reactions in basic media starting from [ReOCl₄]^–. Compounds 1–11 are referred to as ‘equatorial’ because the co-ordination of both bidentate L^– chelates occurs symmetrically in the equatorial plane orthogonal to the MO moiety, with a mutual cis-phosphorus configuration. The site trans to the oxo group is always occupied by an oxygen-containing monodentate ligand the co-ordination of which determines the stability of this class of octahedral complexes. Nevertheless the ^–OR group is easily exchangeable with other nucleophiles available in the reaction mixture by the mass effect and/or donor ability of the incoming ligand. The crystal structures of complexes 1 and 5 have been determined: 1, monoclinic, space group P2₁/c, Z= 4, a= 12.156(3), b= 26.005(6), c= 10.953(2)Å, β= 102.49(2)°, R′ 0.0611 using 1766 observed reflections; Tc–O(1) 1.700(8), Tc–O(2) 1.999(8), Tc–P 2.511(3) and 2.503(4), Tc–N 1.972(10) and 1.976(8)Å; O(1)–Tc–N 105.8(4) and 103.6(4), O(1)–Tc–P 88.1(3) and 88.9(2), O(1)–Tc–O(2) 158.3(3)°; 5, monoclinic, space group P2₁/c, Z= 4, a= 12.056(3), b= 26.303(6), c= 11.005(3)Å, β= 102.32(2)°, R′ 0.0614 using 4752 observed reflections; Re–O(1) 1.692(7), Re–O(2) 2.004(7), Re–P 2.495(2) and 2.493(3), Re–N 1.990(8) and 2.003(7)Å; O(1)–Re–N 104.6(3) and 102.6(3), O(1)–Re–P 89.1(2) and 88.8(2), O(1)–Re–O(2) 160.8(3)°. By treatment of [MOL₂(OR)] solutions with HX (X = halide), further reduction to lower-oxidation-state species occurs when M = Tc, while another class of stable and neutral oxo complexes, termed ‘twisted’, is produced when M = Re. Two mutually orthogonal L^– chelates co-ordinate the metal while still preserving the cis-phosphorus configuration, one ligand bridging an equatorial and an apical position (with the phosphinoamido nitrogen trans to the ReO linkage) and the other two equatorial positions. The equatorial plane is completed by a halide atom. The chloro derivative [ReOL₂Cl] exhibits two crystalline forms, α(12) and β(13), the crystal structures of which have been determined: 12, monoclinic, space group P2₁/n, Z= 4, a= 9.594(1), b= 9.594(1), b= 18.565(4), c= 17.656(2)Å, β= 91.00(1)°, R′ 0.0477 using 2979 observed reflections; Re–O 1.767(7), Re–Cl 2.436(3), Re–P(1) 2.429(3), Re–P(2) 2.479(3), Re–N(1) 1.986(9), Re–N(2) 2.029(8)Å; O–Re–N(2) 162.8(3), O–Re–N(1) 108.6(3)°; 13, orthorhombic, space group P2₁2₁2₁, Z= 4, a= 10.196(5), b= 14.047(6), c= 21.987(8)Å, R′ 0.0671 using 2501 observed reflections; Re–O 1.69(1), Re–Cl 2.422(6), Re–P(1) 2.434(5), Re–P(2) 2.476(5), Re–N(1) 2.01(1), Re–N(2) 2.04(1)Å; O–Re–N(2) 162.7(6), O–Re–N(1) 109.0(7)°. The interconversion between ‘equatorial’ and ‘twisted’ species is discussed. Elemental analyses, FAB mass, IR, electronic, ¹H and ³¹P NMR spectra are reported. The ³¹P NMR signal is diagnostic for both classes of complexes in solution, being a singlet for the ‘equatorial’ and two doublets for the ‘twisted’ species, respectively.

Asymmetrical heterocomplexes containing a terminal technetium-nitrogen multiple bond coordinated to one diphosphine ligand (PNP) and one dithiocarbamate ligand (DBODC), were obtained through a simple two-step procedure under controlled conditions. The resulting complexes [99mTc(N)(PNP)(DBODC)]+ are monocationic, and possess a distorted square-pyramidal geometry where the Tc≡N multiple bond occupies an apical position and the diphosphine and dithiocarbamate ligands span the residual four coordination positions on the basal plane through the two phosphorus atoms and the two sulfur atoms, respectively. Biodistribution data in rats demonstrated that these complexes were rapidly extracted by the myocardium, and retained in this region for a prolonged time. After a few minutes post-injection, lung uptake became negligible, and liver washout was extremely rapid and quantitative. Analysis of heart/liver uptake ratios for these complexes revealed that their values increased exponentially in time, and after 60 min post-injection liver activity was almost completely eliminated into the intestine. Comparison with heart/liver ratios determined for 99mTc sestamibi and 99mTc tetrfosmin showed that values for these latter compounds were approximately 10 times lower than those measured for [99mTc(N)(PNP)(DBODC)]+ complexes at 60 min post-injection. In conclusion, the monocationic tracers [99mTc(N)(PNP)(DBODC)]+ exhibit high myocardial uptake in rats and dramatically high heart/lung and heart/liver ratios, suggesting that this novel class of perfusion agents could be conveniently employed to obtain heart images with superior imaging quality.