The ambidentate substituted cyclam 6,13-diamino-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane (diammac) acts as a sexidentate or quinquedentate ligand to chromium(III). The hexaamine [Cr(diammac)](ClO4)3 was synthesized, as well as isomers of the complex [Cr(diammac)(OH)](ClO4)2, where one pendant primary amine is not coordinated. In the hexaamine complex, the pendant primary amines occupy trans sites, defined by a crystal structure analysis. The [Cr(diammac)](ClO4)3 complex crystallizes in the monoclinic P21/c space group, with a = 16.653 (3) Å, b = 8.793 (2) Å, c = 17.842 (4) Å, β = 114.52 (2)°, and Z = 4. The four secondary amines occupy a plane and the pendant primary amino groups occupy axial sites, forcing boat conformations on the six-membered rings to which the pendants are attached. There is considerable strain in the ligand, and the Cr-N bond lengths, Cr-N(equatorial) = 2.031 (5)-2.055 (5) Å and Cr-N(axial) = 2.067 (4) Å, are the shortest reported for a CrN63+ amine system. This “compression” is reflected in electronic spectra, with the 4T2g ← 4A2g(Oh) and 4T1g ← 4A2g(Oh) maxima (at 427 and 332 nm, respectively) at higher energy than for other hexaamines. The quasireversible Cr(III)/Cr(II) redox potential is also at a very negative potential. Spin-forbidden electronic transitions were also observed at 663 nm (to 2T1g) and 681 nm (to 2Eg). The X-band EPR spectrum of seven lines between 1000 and 6000 G has been assigned for the paramagnetic complex, which shows a room-temperature magnetic moment of 3.87 μB. Two peaks near 680 nm, separated by less than 2 nm, are observed in the powder emission spectrum of [Cr(diammac)] (ClO4)3. Lifetimes in the temperature-independent region below 200 K are relatively long, being 150 μs for the protonated complex and rising significantly to 3.3 ms for the deuterated complex. Emission is observed at room temperature. © 1990, American Chemical Society. All rights reserved.