Binuclear Nickel Complexes with Single Azide Bridges. Structure and Properties of [Ni2(N,N-bis(2-aminoethyl)-N'-(2-pyridylmethyl)ethane-1,2-diamine)2(.mu.-N3)](ClO4)3 and [Ni2(1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane)2(.mu.-N3)](ClO4)3