Synthesis, Structural Characterisation, and Monte Carlo Simulation of the Magnetic Properties of the 3D-Stacked Honeycomb Cs n [{Mn(N 3 ) 3 } n ] and the Irregular Double Chain [{N(C 2 H 5 ) 4 } n ][{Mn 2 (N 3 ) 5 (H 2 O)} n ] Mohamed A. S. Goher, [b] Joan Cano, [c] Yves Journaux, [c] Morsy A. M. Abu-Youssef, [d] Franz A. Mautner, [e] Albert Escuer,* [a] and Ramon Vicente [a] Abstract: Two new polymeric manga- nese-azido systems with formula Cs n - [{Mn(N 3 ) 3 } n ](1) and [{N(C 2 H 5 ) 4 } n ][{Mn 2 - (N 3 ) 5 (H 2 O)} n ](2) were synthesised and structurally characterised. Compound 1 crystallises in the P2 1 /n group and con- sists of a three-dimensional system with end-to-end and end-on azido bridges with the caesium atoms in the holes of the net. Magnetically, compound 1 is a rare case of a three-dimensional net- work with alternate ferro ± antiferro- magnetic interactions. Compound 2 crystallises in the P1 Å group and consists of double chains of manganese atoms bridged by end-on and, the exceptional, (m-1,1,1)-azido bridges. Magnetically, compound 2 shows net ferromagnetic behaviour. Exact fit of the magnetic data was performed for the two compounds by means of Monte Carlo simulations based on the Metropolis algorithm on sets of 10  10  10 (1) and 1  1  320 (2) S  5/2 classical spin centres. Keywords: azido bridges ´ magnetic properties ´ manganese ´ Monte Carlo simulations ´ N ligands Introduction Currently, more and more work in the field of magneto- chemistry is devoted to the study of the magnetic properties of high-dimensional systems; this forces magnetochemists to develop new equations or methods to fit the susceptibility data, to search for uncommon EPR features or to permit the characterisation of cooperative phenomena such as molecular magnet behaviour. In recent years, the manganese( ii )-azido derivatives have provided an exceptional number of unpre- cedented structural and magnetic properties. The main factor that permits such different kinds of properties is the extreme versatility of the azido ligand, which may act as an end-on (m-1,1 or EO) or end-to-end (m-1,3 or EE) bridge to give usually a ferromagnetic (FM) or antiferromagnetic (AF) coupling, respectively. [1] For extended systems such as one- dimensional chains, all kinds of interactions have been found: ferromagnetic EO chains, [2] antiferromagnetic EE chains [2, 3] or alternating FM ± AF systems [2, 4] due to alternating EE/EO bridges. For two-dimensional systems, quadratic layers with EE bridges with AF coupling [5] and also alternating EE/EO systems [6] , which magnetically shows alternating FM/AF coupling, have been characterised. Regular AF three-dimen- sional compounds have also been described with only EE azido bridges. [7] Some authors have introduced a second kind of bridging ligand such as 4,4'-bipyridine or bipirymidine, to obtain complicated two-dimensional or three-dimensional topologies. [8] From our research in this field, we have characterised two new Mn-azido systems with caesium or tetraethylammonium countercations. The size of the cation determines a drastic variation in the structural arrangement: whereas Cs n [{Mn- (N 3 ) 3 } n ](1) is a three-dimensional system that hosts the caesium atoms in the holes of the structure, [{N(C 2 H 5 ) 4 } n ]- [{Mn 2 (N 3 ) 5 (H 2 O)} n ](2) consists of double chains of manga- nese atoms isolated by the large tetraethylammonium cations. The coordination modes of the azido bridges are exceptional for different reasons in each compound: in 1 layers of alternating EO/EE bridges are linked to each other by means of new EE bridges to give an FM/AF/AF three-dimensional system, whereas in 2 the manganese-azido chains (m-1,1 [a] Dr. A. Escuer, Dr. R. Vicente Departament de Química Inorga Á nica Universitat de Barcelona, 08028 Barcelona (Spain) Fax: ( 34) 93-4907725 E-mail: aescuer@kripto.qui.ub.es [b] Dr. M. A. S. Goher Chemistry Department, Faculty of Science Kuwait University, P.O.Box 5969 Safat, 13060 (Kuwait) [c] Dr.J. Cano, Dr. Y. Journaux Laboratoire de Chimie Inorganique, UMR 8613, CNRS Universite  de Paris-Sud, 91405 Orsay (France) [d] Dr. M. A. M. Abu-Youssef Chemistry Department, Faculty of Science Alexandria University (Egypt) [e] Dr. F. A. Mautner Institut für Physikalische und Theoretische Chemie Technische Universität, 8010 Graz (Austria) FULL PAPER  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0947-6539/00/0605-0778 $ 17.50+.50/0 Chem. Eur. J. 2000, 6, No. 5 778