The focus of this dissertation is to study the magnetic properties of several novel transition metal complexes, with particular emphasis on using SQUID magnetometry alongside electron paramagnetic resonance (EPR) and spectral simulations for accurate elucidation of the magnetic parameters: S (effective electron spin), g (Zeeman tensor), D and E (axial and rhombic zero-field splitting energies), and U (energy barrier of magnetization relaxation). The main goal was to examine structure-property relationships in order to search for the key features which give rise to large magnetic anisotropy, and to find new compounds which exhibit long-term remnant magnetization at a single molecule level, i.e. single molecule magnets (SMMs). The compounds presented include four new mononuclear polyoxotungstate compounds with the formula [M(HX[superscript V]W₇O₂₈)2]¹³⁻ (M = Cr[superscript III], Fe[superscript III], Mn[superscript III]) (X = As/P). Although, these compounds contain a simple M[superscript III]O₆ octehedral core, they were found to exhibit unusually large zero-field splitting (ZFS) due to imposed molecular distortion from highly charged W[superscript V] and P[superscript V]/As[superscript V] ions in the second coordination sphere of the M[superscript III] center. Also presented are four new coordinately unsaturated cyclic alkyl amine carbene compounds. Determination of the oxidation state of the metal center in these two and three-coordinate compounds ,which is vital in understanding their magnetic and chemical reactivity properties, was achieved. Additionally, slow magnetic relaxation, which is a distinctive feature of SMMs, was observed for the iron-based molecules. Lastly, the magnetic properties of three heterotrimetallic M₂Cr(dpa)₄Cl₂ (M = Cr₂, Mo₂, W₂) compounds (dpa = 2,2'-dipyridylamido) are presented. These S = 2 compounds also exhibit slow magnetic relaxation and show increased ZFS upon substitution of heavy ions into the linear trimetallic chain. Additionally, the ground state spin result is confirmed using molecular orbital analysis by density functional theory (DFT) calculations. The reoccurring theme, which relates these very different compounds, is that each possess significantly large axial ZFS (D), an indispensible parameter for the development of spin technology devices. Although significant progress has been made in recent years to better understand the origin of D, so far, the conditions that determine the sign and value of D are not fully understood. SQUID magnetometry and variable-temperature-variable-frequency EPR were used to precisely measure D here, as these are the most reliable experimental sources for studying magnetic anisotropy in molecules with large D. Through detailed experimental and theoretical analysis, the origin and nature of ZFS, in the aforementioned compounds, is described herein. Thus, this dissertation represents a major effort to quantify D in transition metal complexes and to highlight the different factors which can lead to large values of the parameter. It is hoped that this dissertation may serve as a brief guideline for the ways in which future magnetically anisotropic materials can be developed.