Tris(1,10-phenanthroline)manganese](2+) 1,2;1`,2`-trans-bis [nonahydro-closo-decaborate] (23) bis(dimethylformamide) containing the complex anion [MnPhen 3 ] 2+ with the paramagnetic central Mn atom and the [B 20 H 18 ] 2! anion formed by fusion of two closo clusters B 10 H 9 was synthesized and characterized by single crystal X-ray diffraction and by IR, X-ray photoelectron, and Raman spectroscopy.This study continues our investigvations concerning physicochemical characteristics of compounds based on polyhedral boron hydrides, in particular, their magnetic [1] and nonlinear optical [2] properties. Previously [1] we revealed weak ferromagnetic interactions at 20 K in the complexes [MnPhen 3 ][Co(C 2 . B 9 H 11 ) 2 ] 2 and [MnPhen 3 ][C 2 B 9 H 12 ] 2 [Phen is phenanthroline, 1,10-C 12 H 8 N 2 ; C 2 B 9 H 12 is the dodecahydro-7,8-dicarba-nido-undecaborate(13) anion] containing the paramagnetic Mn(II) ion (3d 5 ), which is a new fact in the chemistry of boron hydrides. Here we report on a compound containing the 1,2;1`,2`-trans-bis-[nonahydro-closo-decaborate](23) anion, [B 20 H 18 ] 2! , belonging to the scope of chemistry of macropolyhedral clusters [3,4]. Active studies of the synthetic potential of this anion and its derivatives were initiated in the 1990s within the framework of a search for boron-containing materials suitable for neutron chemotherapy of cancer. Combination of this anion with a paramagnetic center opens new research prospects, and properties of such compounds are of doubtless interest.[Tris(1,10-phenanthroline)manganese](2+) 1,2;1`,2`-trans-bis [nonahydro-closo-decaborate](23) (hereinafter, [MnPhen 3 ][B 20 H 18 ]) was prepared as described in [1] by precipitation of the anion with a bulky cation in a weakly acidic aqueous solution: [MnPhen 3 ] 2+ + [B 20 H 18 ] 2! 76 [MnPhen 3 ][B 20 H 18 ]2.