Studies on cobalt(II) complexes with N,N,N′,N′-tetraethylpyridine-2,6-dicarboxamide(L1) containing PF6-,BF4-,ClO4- and NO3- anions: X-ray crystal structures of [Co(L1)2(CH3CN)](PF6)2 and [Co(L1)2(H2O)2](X)2·H2O(X=PF6-,ClO4-)