Distortion isomerism in a thiocyanate-bridged copper(II) dimer. X-ray structure of .beta.-bis(.mu.-thiocyanato-N,S)-bis[bis(5,7-dimethyl[1,2,4]triazolo[1,5-a]pyrimidine)(thiocyanato-N)copper(II)] and comparison of its spectral and structural properties with those of other copper(II) 5,7-dimethyl[1,2