Comparison of free and metal coordinated 1,4-disubstituted-1,4-diaza-1,3-butadienes. Crystal and molecular structures of 1,4-dicyclohexyl-1,4-diaza-1,3-butadiene and trans-[dichloro(triphenylphosphine)(1,4-di-tert-butyl-1,4-diaza-1,3-butadiene)Palladium (II)]