Sperling, 62, joined the company in 1996 and spent nine years as its chief executive. He is credited with building a real estate investment, development and management portfolio for the company such that it could exist beyond 2014, which is when the long-term Downtown Seattle real estate partnership that gave the company its start 60 years ago is set to expire.

Alfred Glancy, executive chairman of Unico Properties and Unico Investment Group, said in a prepared statement that Sperling's drive "gave the company life after 2014." Glancy chose not to be interviewed about why Sperling resigned and authorized a spokesperson to say only that the decision was "mutually agreed upon."

Reached at home on Bainbridge Island, Sperling tells GlobeSt.com that it has been "a terrific 13-year ride," that he's "enjoyed every minute of it," is "proud" of what he's accomplished in the way of portfolio growth, customer service, innovation and green buildings, and is "looking forward to new challenges." As for why he left, Sperling also declined to disclose the events and discussions that lead to the decision.

"I've got other things to do," he says. "There are a number of things I'm involved with in the community, both business and charity; I plan to stay engaged in Seattle."

Unico got its start in 1953 when it entered into a ground lease and partnership with the University of Washington in 1953 for the development and operation of the six-block Metropolitan Tract. The six-block area bound by Union and Seneca streets and Third and Sixth avenues is now home to Rainier Tower, Puget Sound Plaza, Financial Center, IBM Building, Skinner Building and Cobb Building.

As part of that initial deal, Unico agreed not to have financial investment in any real estate projects inside the city limits of Seattle to protect the University's interests in the Tract. However, Unico had its hands full simply developing and managing the Metropolitan Tract for the first 40 years. It wasn't until Sperling arrived that the company began building a portfolio of its own to ensure the company would have something to do if the University selects a different partner come 2014.

By 2002, Unico and the University had determined that the real estate market had evolved and diversified to such a degree that it was now possible for Unico to pursue projects in Seattle that do not compete with the Tract. A new agreement was signed that shrunk the boundary, allowing Unico to manage, lease, develop and invest in real estate opportunities as long as they were outside of the CBD--defined as the area North of Stewart Street and South of Dearborn, from Interstate 5 to the waterfront.

Sperling, who brought with him to Unico 35 years of corporate finance and real estate expertise, was instrumental in Unico's acquisition of nearly 4.8 million square feet of Class A urban office properties and expansion into the medical office and apartment markets. During his tenure, the company completed $1.5 billion in investment sales, acquisitions, and corporate services assignments.

Unico's office portfolio now totals more than 6 million square feet in 30 buildings in Washington, Oregon, Northern California and Colorado. Investments in assets outside the Metropolitan Tract include 100 Pine St. (402,500 square feet) in San Francisco; US Bancorp Tower (1.07 million square feet) in Portland, OR; and Key Center (478,000 square feet) and Skyline Tower (408,460 square feet) in Bellevue, WA; and 1777 S. Harrison St. (205,400 square feet) in Denver.

The company has been ahead of the game in terms of green buildings. Its 100 Pine building in San Francisco was the first multi-tenant office building in California to earn LEED EB certification. The company also has received LEED Silver certifications for two new development projects the Puget Sound region and is seeking LEED EB certifications for seven existing buildings in the Pacific Northwest. Ten of the company's buildings have earned Energy Star certification and LEED NC (New Construction) certification is standard operating procedure for all of its development projects.

It's also ahead of the game in terms of customer service. In 2008, tenants' rankings of Unico-managed buildings placed Unico in the top 1% nationally among 6,000 comparable buildings surveyed, earning it a "Best in Class" distinction from CEL & Associates, a national, independent real estate consulting firm.

Kuhrau, the new CEO, joined Unico the same year as Sperling. As chief investment officer and senior vice president, Kuhrau was responsible for investment and development for the firm. "Quentin shares our firm's mission to ensure that Unico's foundation and future growth is based on making its employees successful, delivering best-in-class customer service, developing enduring partnerships, and enhancing long-term, shareholder value," said Glancy in a prepared statement. "Quentin has demonstrated the vision and experience that has successfully led the company through market cycles."

Sylvester joined the firm in 2003 as manager of investments and was promoted to VP of investments in 2004. At Unico, Sylvester has been the go to guy for financing, acquisitions and dispositions for its medical office and office properties and its development projects. He also is leading the development of inhabit, Unico's eco-friendly, modernistic modular units that can be linked together to form multiple-story, in-city apartment communities. "Jonas' leadership efforts in diversifying our firm into new properties and markets have been key to Unico's growth and expansion," said Glancy.