Four cyclometalated Pt(II) complexes, [PtL(L')][ClO4] [HL = 6-phenyl-2,2'-bipyridine; L' = pyridine (1), 4-aminopyridine (2), 2-aminopyridine (3), 2,6-diaminopyridine (4)1, were designed and synthesized to probe intramolecular N ... Pt interactions. The crystal structures of the compounds show that the pyridine ligands are almost perpendicular to the planes of the molecules. In addition, the pendant NH:! groups of the 2-aminopyridine and 2,6-diaminopyridine ligands are close to the metal centers in complexes 3 and 4, with the Pt-N(H-2) distances (3.065-(3)-3.107(3) Angstrom) significantly shorter than the sum of the van der Waals radii of Pt and N. These compounds were also studied by electronic spectroscopy. All the complexes display intense intraligand pi --> pi* transitions at 200-340nm (epsilon = 10(4)-10(3) M-1 cm(-1)) and moderately intense (epsilon similar to 10(3) M-1 cm(-1)) metal (Pt)-to-ligand (pi*) charge-transfer (MLCT) transitions. For 1 and 2, the MLCT transitions occur at similar to 390 nm, but the MLCT transition of 4 is exceptionally low in energy (492 nm). The low-temperature emission spectra of the complexes in frozen EMD glass indicate that (3)pi pi* is the emissive excited state for 1 and 2 but the emission of 3 is from a (MLCT)-M-3 excited state. On the basis of the spectroscopic results, the order of energy of the MLCT excited states is established as 1 similar to 2 > 3 > 4. It is proposed that the red shifts of the MLCT transitions in 3 and 4 are due to increased electron-donating abilities of the ancillary pyridine ligands and intramolecular interactions between the orbitals of amine nitrogen lone pairs. Crystal data for the complexes are as follows. 1: triclinic P (1) over bar, Z = 2, a 8.7917(2) Angstrom, b = 10.6398(3) Angstrom, c = 11.9592(3) Angstrom, alpha = 107.130(1)degrees, beta = 92.522(1)degrees, gamma = 111.509(1)degrees. 2 . CH3CN: triclinic P (1) over bar, Z = 2, a = 7.0122(4) Angstrom, b =12.9653(8) Angstrom, c = 14.0283(9) Angstrom, alpha = 107.3100(10)degrees, beta = 102.7640(10)degrees, gamma = 91.6320(10)degrees. 3 . CH3CN: triclinic P (1) over bar, Z = 2, a = 7.6459(1) Angstrom, b = 10.8433(1) Angstrom, c = 14.8722(2) Angstrom, alpha = 99.383(1)degrees, beta = 93.494(1)degrees, gamma = 101.385(1)degrees. 4 . CH3CN: triclinic P (1) over bar, Z = 2, a = 7.862(2) Angstrom, b = 10.977(3) Angstrom, c = 14.816(5) Angstrom, alpha = 99.34(2)degrees, beta = 92.64(2)degrees, gamma = 104.11(2)degrees.