Complexes of MoO2Cl2 and WO2Cl2 with the ligands N,N,N′,N′-tetramethylethylenediamine, N,N′-di-t-butyl-1,4-diaza-1,3-butadiene and N,N′-dimethylpiperazine. cis,trans,cis vs all cis coordination in bidentate N-donor complexes of WO2Cl2 and MoO2Cl2. Crystal structures of MoO2Cl2(N,N,N′,N′-tetramethylethylenediamine) and MoO2Cl2(N,N′-DI-t-butyl-1,4-diaza-1,3-butadiene)